Gateway 832GM OVERHAUL
$350 wouldn't really be enough to upgrade to Core 2 Duo ($170ish for processor, $100-130ish for motherboard, $150-200ish for memory, plus more if your graphics card is AGP...), so I guess Pentium D and more memory is the way to go for now. However, I HIGHLY SUGGEST waiting if you think you can spare a bit more cash in the future ($500 will get you a nice Core 2 Duo setup).
Depending on what resolution you're running, the graphics card might be your biggest issue. If you're doing 1024x768, you'll be fine. 1280x1024 might get a little iffy, and 1600x1200 is pretty harsh on Oblivion for a 6600GT.
So, basically, is that PCI-E or AGP?
We're all going to die someday, regardless of race, gender, beliefs, or any other differences.
The where, when, and how are things that we can try to control, but in the end, it's the mark that we leave on this world that we truly have power over.