Either a HD 6970 or HD 7870, used of course and on Craigslist. I got my 2x HD 6970's for $220 and do 1000 kh/s. That's cheaper than buying a HD 7950 for $300 to $450 that does only 550 to 700 kh/s.
Where did you got 2x 6970 for 220$ ?
Read his post, and you'll find out.