Buy what ever you can for as close to $300 each as possible. I've found that anyone that I've gotten is getting with in 1Mh/s or less of any of the other ones - all above 30Mh/s (right about 31Mh/s) single mining and all right in the 29.5-30.5Mh/s dual mining. I've got XFX, MSI, Asus, and PowerColor. All of them are stable at 1200 CPU / 2150 MEM. I could probably tweak them all a little bit on voltage and speed, but the down time doesn't make it worth it.

The 570 is also a great buy, the one XFX one I have is running at 28.75 Mh's while dual mining. Honestly probably the bet ROI, out of all the cards - but I got it for $229.