With $400 dollars you would be better off waiting until the new 69xx series card come out from ATI,should come out end of this month or begining of next.If you have any extra money after that you might want to consider an Eyefinity setup(running more than 1 monitor)looks amazing.
There is a new nvidia card out 580gtx but it is just out of your budget.Right now if i had to buy i'd buy a 5870.But the 6870's do very well in crossfire.Since you don't play very demanding games i would get a 6870 and then crossfire in the future.You should really wait like 3 more weeks for those 69xx series it will become the best bang for your buck and i'm sure prices will drop for the 58xx series making it more easier to crossfire a 5870.Really is a bad time to want to buy a graphics card right now with new cards from both sides prodcueing more cards right aorund the corner.
Its Christmas, its not a bad time, everyone wants to treat themselves this time of the year. Buy Nvidia, the gtx 570 should be out this week.
Talk of 349.99
AMD dropped the ball this year, there cards are late. Now there is talk of more delays, and no offical date has been confirmed. So to actually get a AMD (6970,50) before Christmas seems doubtful, imo.
edit: I don't often suggest waiting, because after the next new card, there is always something else coming.
The 2 card route has the 6850/6870 - gtx 460 has BEST options. I have 2 gtx 460's. Its awesome, but you have to tune your system with dual cards more. Make sure your getting the most out of them. Driver/profile/setting wise. Things that end up frustrating some users.