My e6750 (an intel dualcore) works just fine in Ubuntu.
Btw - I think dual core cpus really are the sweet spot for gaming right now. You see, every core is powerfull enough to handle even those games that are not yet fully optimized for multiple cores, while the one that isn't being used by the game easily handles all the stuff related to windows, your processes and applications that may be running in the background.
Sadly, I could not find any chart comparing a decent dualcore to your cpu this time - I suspect that may be because they are too far apart in terms of performance, but there may be such a chart out there, I'll edit it in if I manage to find one.
Anyways, and I can't stress this enough: It does not make sense to invest into a 8800gt if you're planning on keeping that cpu. You're basically adding racing tires to your lawnmower - ok, a bit drastic, but you get the picture. I suspect the 8600gt would also be overdimensioned.
So. here's what you can do right now...
1) Fuck it all - save up and get an entirely new pc once you can while you replay good old games. Get a good pc (cpu wise) with a crappy graphics card first, make sure the mainboard is decent. Then add RAM and a graphics card as funds allow. Possibly your best bet, if saving up is your thing. I know it isn't mine.
2) Get a decent cpu. That would mean an Athlon x2 5000+ to make use of a 8600gt or something like my E6750 (which had an awesome price/performance ratio back when I got it) or current equivalent for the 8800gt. However, that also means a new mainboard, and once you go AMD or INTEL there's not really any way back on that one. Still, a good cpu and a decent mainboard won't set you back that much more than a 8800gt. Keep in mind that your ram sticks need to be compatible with the mainboard, too. Oh, and did I mention you'll more than likely need a new power supply unit? That's basically why I kind of lean towards 1.
3) Get the graphics card. There's a good chance you may still need to get a better PSU, but maybe yours is ok - I'd need to know your specs to tell for sure.
Also, make sure you actually have a pci-e slot - I know the 6600gt had an AGP version.
Anyways, as I mentioned before, you may feel better about your lawnmower, but you still need a decent cpu before it behaves like a racing car. Actually, if you insist on keeping your current cpu - I'd propose looking into a 7800xx series card, if you can find those somewhere.
Whichever route you take, feel free to ask if you have any questions or would like some more specific advise.
Edit: Ok, i found one and I "enhanced" it a bit for your viewing pleasure. Yes, I also fear my mad MSPaint skills. Note that these are rough estimations.
- hope your eyes don't bleed too much, I really didn't check the writing after converting it to jpg. Should have done it outside paint.
