A Nvidia GTX 760 or AMD 280X should fit the bill. The 280X costs a little more(right at $300 if you shop around) with a little more performance. Both are as fast or faster than 2x 6870 in crossfire. I'm not a big fan of crossfire on low end cards especially older low end cards. I don't think you will be as happy with the two 6870's in crossfire. Do yourself a favor and spend the $300 on a new single card. If you want to stretch a hair over $300 or watch for sales during the holidays you could pick up a GTX 770 (They can be had today for $330).