look for an AM2+ mobo (the am3 just adds DDR3 cost thats mostly pointless)
in that price range you should be able to fit either a single powerful video card, or a medium power dual card setup. Which one you do depends on how long you are keeping it. If you're gunna keep it for a long time, then just grab a GTX280/285/4890 and SLi/CF it later when you get the cash. If you only plan on 2-3 years, then just grab a couple 4870s/4850s/GTS250s/GTX260/GTX275 depending on how much you're willing to pay.