Under that price ceiling - EVGA GTX 480 for $219.99 (new - no rebate). Just went out of stock - you'd have to wait a couple weeks for shipping.
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?
EdpNo=2148469&CatId=3669
Amazon has the same card in stock for $249.99. Its powerful, but hot and power hungry. If you can't handle that a GTX 560 Ti 448 can be had for under $300 and is just about as fast, but cooler and more efficient.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130738
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130758 (I think this would be the fastest at $300).
I didn't include cards that need rebates to achieve that price limit - you still have to cough up whatever their charging and the money is out of your hands for months.