So I've been researching the parts out there right now. The problem is this: The hardware sites benchmark this stuff with high-end CPU's. Sorry, but even if I upgrade my CPU and motherboard (an inexpensive thing to do these days), I'm not going to buy an Athlon 64 3800.
Right now it looks like I can upgrade my CPU and motherboard to something like a mobile Athlon XP 2600+ (over-clocked lots) for around $150. I don't know what kind of impact that will have on my existing video card, but I'm guessing it won't be great. Video card upgrades, those that aren't little incremental upgrades, pretty much rest with the Nvidia 6800's ($300) and the 6800 Ultra's ($400). I'm not really that geeked to spend that much.