Hi @Givememoney,

You'll only see a 17% increase in performance according to game-debate - barely noticeable, if at all.  However, if you upgrade to an Intel Core 2 Quad 9650 3.0 GHz, you'll notice a 76% increase.  Going from a dual to a quad is much better step.  I bought mine used off of eBay for $41 and it came in brand new condition.

Increasing RAM also increases performance.  Modules need to be same capacity, brand, and MHz.  So I don't know how you have 10GB of RAM in a 780 with 4 slots.  RAM max is 16GB.  In this case, RAM could cost you more than the CPU.  Kingston Value Brand (KVR) rarely works in Dell's.  The volts can be off in off-brands.

An SSD also breathes new life into an older machine.

Keep in mind if you were to do all of the above, you're closer to affording a newer, refurbished MT that can take GPU's better than an SFF can.

The rest is up to you.  Depends on what you all like using your PC for and what games you like to play.

It doesn't work like this in all cases, but in this case, I'd upgrade to the quad CPU first - You will have a fast machine.  Also, Amazon has SSD's for less than $41.

You can't.  It's for system operation.  I'm sure you noticed HDD's are the same way.

RAM sticks matching size is normally a technical requirement for the 780.  However, if yours is working, not a problem.

Optiplex 780 Tech Guide.pdf

Hello thanks for the info I am gonna get the cpu u mentioned and a GT 1030 LP for my pc. And u said ur wondering how I have 10 gbof ram? I have 3 2gb sticks and 1 4gb stick. But, when I go into my control  panel, it only says that I have 9.83 usable?? How can I fix this?? 

Ok, thanks. and you are sure that this cpu will fit in my system, right?

100%.  Not only is the Q9650 in the tech guide, it's what I did with my 755 MT.  I strive to be as accurate as possible.