Hi @jibjab123123123123,

Let's answer you question first. Check out this thread about discussion on Optiplex 780 MT memory support, which is 4x4GB=16GB maximum, i.e. 2x4GB is ok. Noted that if your Windows is a 32bit version, only 4GB memory with be in use. Supported DDR2 memory type as table below:

Moreover, the game's lag problem may not lead by lack of memory. Look into Robloxs system requirement:

Your 780 installed with 4GB memory and a Core2Duo E8400 CPU which manufactured in 2008'. Both of them already met the game's hardware requirement. Maybe GPU is the root cause instead. Are you using the on-board display which is "Integrated Intel GMA 4500"? It's just a general GPU but not a great performer. If you just plan on web browsing and web games, you will be just fine with it. But for 3D games like Robloxs, better get a proper display card. 780 DT contains 1 PCIe 2.0 x16 slot with full height card supported. Common PCIe display card should be fine with your system.

Furthermore, here's a video showing you how to configure the game to reduce lagging. See if it helps.

Dell Optiplex 780 Tech Guide.pdf - if it helps.

If you use a 64-bit version of Windows, a max of 16GB ram can be utilized in your 780 DT.  32-bit Windows only utilizes a max of 4GB.  If using non-Dell brand memory, Crucial is recommended as it's guaranteed to work in Dell's.  You're on the right track with asking about volts.  Other brand RAM doesn't work if the voltage isn't right.

New RAM may improve performance some, but not as much as if you put a quad cpu in it.  The max CPU for the 780, Q9650 Intel Core 2 Quad 3.0GHz, improves performance 64% over the Duo you have.  They come as refurb and used on eBay.  If you find it a little pricey (around $40) (new is much, much more) for a PC your kid is using, a 2.83GHz, 57% faster, brings the price down around $10.  %'s are according to game-debate.com.  Credit to JOcean for helping me discover it.

I plan on putting a quad in my 755 until budget allows me an Optiplex with an i7.  Mine seems to "bottleneck" backing up media files to flash drive and is almost as slow as USB1.0 as it compresses the files.  The other way around, then it's fast.

If you want, you can also compare computer builds at userbenchmark.com.  Credit to bmcowboy for helping me discover that one.

@bradthetechnut Anytime dude ^^"

I think @bmcowboy has made an error, I'm pretty sure the Optiplex 780 desktop uses DDR3 memory, not DDR2.

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-780_setup%20guide2_en-us.pdf

Dell specs indicates it will take any non-ECC 'desktop' grade 1066 MHz DDR3 (PC3-8500) SDRAM modules up to 16GB

If you have trouble finding PC3-8500, try PC3-10600 or PC3-1280 which will clock down to slower 1066 MHz.  (Plenty used SDRAM on eBay, make sure it is non-ECC for 'desktop' use and of reputable brand such as Crucial, Hynix, Samsung etc)

eg. if you visit Crucial.com, you will see the only SDRAM products they sell for Optiplex 780 are rated: 

DDR3 PC3-12800 • CL=11 • UNBUFFERED • NON-ECC • DDR3-1600 • 1.35V • 512MEG X 64 •

Only 64 bit Windows will benefit from the extra RAM.

As already suggested, try a discrete low profile graphics card if you are using the Intel GMA4500 internal graphics.  Have  you tried lowering the settings within the game to try and reduce lag?

I googled this youtube video, running Roblox on E8400 which may be of interest

https://youtu.be/F69ofHZn3Og

Thank you for the correction. I mess up info between 745 and 780. Sorry for the mistake.

My last post above was edited to include a price.  So with 3 upgrade options (naturally), the rest is up to you, @jibjab123123123123.  I also saw the above YouTube video.

Kingston Value RAM (KVR) is one to avoid as it almost never works in a Dell or doesn't work properly.