I saw that no one had answered your message so here are my thoughts. I doubt that you need to upgrade your memory to get it to work but you sure would want to. I bought 3x 2GB memory upgrades to the 2 GB I had. A total of 8 GB is nice to have and memory is not that expensive any more.
About the graphics, I am not a gamer so I can't tell for sure but I can tell you what worked for my applications. I installed an Nvidia Quadro FX 4600 that I bought for $240 (new, amazon). I did not have to upgrade the PSU and I have not seen any side effects (overheating or noise). However I do think about getting a more powerful PSU eventually. I use this card for quad buffered stereo under Linux and it is amazingly fast. The FX4600 should also perform well when used with Games - after all it is a high-end graphics card. I think this card is the best deal (that I could find) for the price/performance ratio. The FX 3800 costs almost the same as the FX 4600 so the lower end is not worth it and the next step beyond is already in the ~> $800 range (FX 5600).
Okay some corrections - I dealt with 2 upgrades that I confused a bit so here it comes ....
The current system: Precision 390, Intel Core2 6400 2,13 MHz, 4 GB Ram (sorry the other machine was upgraded!), Nvida Quadro FX4600, Nvidia 3D Vision 2 kit, Monitor SamSung 2233Rz, 1.5 TB system disk, original PSU, motherboard.
OS: CentOS 6.2
Hope this helps.
Just in case anyone else wanders to this thread in the same way I did.
LesserTX - I know I'm a little late here, but the answer is yes, it was to be expected.
The Dell 0DN075 motherboard does NOT support the 'x8xxx' range of socket 775 processors. I learned this in the very same way that LesserTX did - I currently have an E8500 sat on my desk, and once installed, the Precision 390 does a full, constant, initial power ON (All fans, inc GPU fan at FULL, all indicator lights ON).
So there we have it. Back to the drawing board, and back to my E6600. And if anyone's in the market for an Intel E8500 3.16GhZ C2D chip, there's about to be one on eBay UK!