What video card do you have currently? By the "next generation of games" do you mean Doom 3 and HL2? Because let me tell ya, I've got an ATI 9700 Pro and it's gonna run the dogshit outta both of those. With good graphical settings across the boards. And at a decent (1024*768) resolution.
You might ask, Blitzkrieg, how can you know that your system is gonna run these games at said resolution and settings well? I can guarantee ya, no
test told me how my system is gonna do. Unless you've run one of the "demos" for these games, be it the leaked alpha for Doom 3 or the leaked graphical demo for HL2, and know for a fact that your computer is gonna blow on it, then don't take the word of anyone on what you NEED to run this game well. It's funny that I say all this and expect you to trust what I say, however, I will say that any "requirements" you hear for these games are absolute horseshit. You DON'T need a fucking X800 Pro or a 6800 Ultra to run these games good. Period. Amen. The only application that would EVER require a card of that magnitude is Unreal III. The guys who are developing it specifically said, "the only card that's managed a decent frame rate on this engine is the 6800. Most times we'd get about
1 frame per second, but with the 6800 we get an average of 30 FPS."
If you have that card, a 9800 Pro or just about anything from the last generation of Nvidia (5200 excluded), you'll be able to run these games without much trouble. Hell, even a 9600 Pro or XT should be able to do these games justice.
However, you may have not meant either of those games, and you might be talking about something even further off in the future. The only situation that would require such a card for decent performance is the one I mentioned earlier, Unreal III's engine.
I'm not gonna answer about how much of a performance increase you'll get until you tell what your current video card is.