That's pretty beefy. I'd go with the Nvidia, myself. Longer shelf life, since the rest of your setup is pretty much a dead-end technologically. Not that there's anything wrong with it, but the next step for you would be the new dual-core chips, and that would require you to get a new MB, new RAM, new CPU, and new video card.
If you don't want to upgrade anything else in the next year+, then the extra $$ in the video card now is a decent investment.
I gotta ask, though, what video card do you have now? It the current board is decent enough, it may not be worth upgrading at all.