You could also get a gtx670 without FTW for $399, but that's pushing your budget limit. And I suspect it might be overkill, but if you think you need to upgrade that much and are willing to push your budget just a tad, it could be worth it.
Thank you for the fast response. I am using a 1920x1200 widescreen, on a 28-inch monitor. My heart is not set on Radeon, I just figured it was better for AMD. But I will gladly pick whichever is the better choice.
The higher-end card out of the two I just named is the gtx670. Geforce cards are certainly compatible with AMD mobos/CPUs. Whether one is preferable over the other, I'm not sure. I've always been an intel kind of guy.
But at 1920x1200 on a monitor that size, the gtx670 won't be overkill, but it definitely isn't necessary either. "Better" choice depends on if you think the increase in quality is worth the money.
If you want the highest quality card near your $400 budget, be prepared to shell out $400-$420 on that gtx670.
If you take a step down in price range, the best card is the Radeon 7950 (check link above for prices).
It's up to you to decide which money range you are more comfortable with. But you don't need to worry about compatibility!
EDIT: The only disclaimer is that putting a geforce card on an AMD board will kill your chances of adding a second geforce card in SLI later. If that is a problem for you, go with Radeon!
While trying to build a new PC I have been looking at a lot of parts and prices ... I have yet to find anything selling cheaper on TigerDirect or CircuitCity websites ... even with the emails I get full of "incredible" deals.