For full HD gaming (1080p) the graphics card to buy in today's market is the ATI 5850. Price wise though, that is going to put you around $300 US at least. But it'll take an ATI 5850 to give you Good performance @ 1080p with high graphic settings (Of course this also depends heavily on what you're playing).
Here's an ATI 5850 for $285 (If you're in the US)http://www.newegg.com/Product/Product.aspx?Item=N82E16814150494&cm_re=5850-_-14-150-494-_-Product
If that's out of the budget, you could consider an ATI 5770 ($155) or ATI 5830 ($215) as less expensive alternatives. You could run a Crossfire setup with 2 ATI 5770's, but for the money I'd rather have a single ATI 5850. Generally, I'd rather have the best single GPU I can afford, rather than two lesser cards in Crossfire or SLI.