It sounds like you're mainly into multiplayer shooters. Luckily they tend to be more GPU-bound than, say, an RTS game.
If I were you, I'd pick up an 8800GT and nothing else. In about 18 months the new Intel Nehalems will likely be available at mass market prices (and perhaps AMD will have gotten its act together). That will be the prime time to make your full upgrade, and you can just move the 8800GT over to the new system. At least that way you won't be throwing away money on a cheaper, temporary video card.
Until then, I wouldn't waste any money delaying the inevitable. Your motherboard is outdated, you'll need new RAM, you'll want to upgrade to Vista in a bit, and when you toss in a new CPU and better GPU, you've basically rebuilt the entire PC. Best to wait it out, get a better bang for your buck next year.