Dell Optiplex 3020 SFF CPU Upgrade!

Same generation so should be OK. In fact you might get away with installing a 4th gen i7!

Whether your system would actually improve in performance is a different matter. Remember that the CPU is only one aspect of the power, many applications are limited by memory bandwidth and/or disk throughput and/or network bandwidth rather than raw CPU power. Try running the Resource Monitor tool (in Win 7 it hides in All Programs / Accessories / System Tools) & find where you bottleneck is before throwing resources at upgrading the CPU.

Do remember that a more powerful CPU will increase the load on the PSU and consume more power.

The humble i3 actually does quite well unless you're into heavy, heavy numbercrunching or multitasking.

Top listed CPU:  Core™ I5-4570 Processor (Quad Core, 3.20GHz Turbo, 6MB, w/ HD Graphics 4600), 84W.  If you were to try an I7-4790 for example, it's only 84w.  Regular size print:  Not recommending as it's not in Dell specs, but your 3020 is in a generation that could exceed some specs.  If a socket type is the same (likely is), and chipset it goes with is the same, there we go.  Neither CPU will strain your PSU.  PSU is made for max CPU with some power to spare.

Optiplex 3020 Tech Guide 

You will get a definite notice in speed if you go from an HDD to an SSD if not already.  It's also a power saver, needing only 5v vs. 12v for an HDD.

You can also pit CPU's into game-debate.  For example, Google "I3-4130 vs. I5-4570 game debate"  Tap on game-debate link.  Scroll down some, and it shows improvement in performance.  Tap on a CPU score for some ideas of what it can do compared to your old CPU.

A better example is when I used I5-4590 for the newer CPU.  Game-debate gave me 65% better performance.  So it's not 2x as much, but better than nothing.  Unless app is CPU or RAM heavy, you might not notice it too much as compared to SSD.

That's the experience I had when I went from a dual core E8400 to a quad Q9650 in a 755.  But it did handle recording studio software better once I also replaced and increased RAM.  Nearly the same for my 7010.  It has max listed CPU, but increased in speed using an SSD.  Going from 8GB to 12GB RAM yielded no speed difference, but can it ever handle multiple apps including multiple music windows including recording studio windows.

If you heard RAM can also increase speed, it's noticeable if the RAM is below 8GB.  Might not see anything beyond 8GB unless app or game is RAM heavy or using multiple apps at once.

P.S.  I know you said "I3-4160,".but I couldn't see your post while typing mine.

So how's all of that?

Adding more RAM won't help if your system already has enough.

Changing HDD to SSD generally makes the system boot much faster but, unless your applications do a lot of disk I/O - or you are running out of physical memory & therefore needing to use the swap file - you won't notice much difference in performance. (Lower power consumption is a good reason for switching from HDD to SDD but note that the extra power used by HDDs is drawn from the +12V bus & most systems have massive overcapacity here, being more critical on the +5V & +3.3V buses)

The key to improving performance is finding the bottleneck & in Windows the "Resource Monitor" tool can be very helpful. Standard & gaming benchmarks might not be measuring the same thing as your usage profile. Rockstars, yes I'd expect media editing to be very heavy on RAM, quite demanding on disc but probably not very sensitive to CPU power, unless you're needing to encode a lot of video...

I differ when it comes to SSD's.  Experiences may vary, but in mine, SSD's speed overall performance.  If all it did was shorten boot and shut down times, I wouldn't be impressed.  Some wait times I had with Windows Media Player is gone.  I don't have to wait for PC to get settled even after desktop appears.  File access is faster.  So on and so forth.  Gamers like SSD's for improved performance, and sometimes store them on a 2nd HDD.

Of course, an SSD won't speed up an app that's already running fast enough.

Also, WD SSD Blue 250GB:  Sequential read speeds up to 560MB/s and sequential write speeds up to 530MB/s.

While SSD read and write speeds are faster, especially NVMe SSD's, I couldn't find read and write speeds for a WD HDD Blue 250GB, let alone.a higher capacity model.

Those sort of speeds are typical of SSDs attached through SATA interface ... don't forget that SATA III has a maximum throughput of 6 gigabits/sec which on the serial interface means 0.6 gigabytes/sec.

The better SATA SSDs are distinguished by a longer life in terms of write operations & more sophistication in spreading load around so individual blocks don't "wear" more than others tends to result in a slower write speed ... in general the bigger difference between read & write speeds is, the longer you can expect the drive to last. The WD Blue SSD  is a low end product (like WD Blue series hard drives) & this is reflected in the small difference.

The data sheet for the WD Blue SSDs is here: https://documents.westerndigital.com/content/dam/doc-library/ja_jp/assets/public/western-digital/product/internal-drives/wd-blue-ssd/user-manual-wd-blue-3d-nand-sata-ssd.pdf

Don't get me wrong, I'm a great advocate of SSDs in lightly used systems which are frequently turned on & off ... especially laptops where the reduced load on the battery is a big plus ... and they're getting much more competitive in price ... whilst conventional hard drives are approaching limits set by quantum physics.

There are several tuning tricks you can use to speed up a HDD - a start is to make a contiguous permanent swap file (and hiber file if you use sleep modes) close to the start of the drive where the throughput is highest, next avoid filling the drive over 75% as you will be using the slow part of the drive (near the spindle where there are less blocks per cylinder) and finally defragment frequently. SSDs do not benefit from tuning in this way as all the space has the same access speed & there is no head seek or rotational latency involved in reading discontiguous files - and should NEVER be defragmented as unnecessary writes will use up the operational life of the unit.

I totally get what you're saying and much I knew about.  I just don't consider WD Blue to be low end.  I believe WD to be a high quality, even though Blue isn't on the high end of their spectrum.  What I do consider low end is Kingston.  In this forum we've had trouble with KVR (Kingston Value RAM) and Kingston SSD's.  This is not to say everybody has had trouble with it though.  Some users have reported good luck with KVR, and some had to replace it.

SSD will improve Game Cut scene loading and system boot time.

They DO NOT Improve FPS significantly for games, compute power for applications, Render time for Davinci Resolve, etc.

If you are wanting smoother game experience Get a GPU. SSD have limited write Life so if your game updates are Gigabytes every month or less I would not advise running on an SSD.  Fallout 76 or World of warcraft tend to have many gigabyte patches and updates for example.

Yah, I know about Red and Black.  They also have Purple.

That's great thanks, that really helps a lot @bradthetechnut .

You're welcome.