Kohd, is correct. Too many unknown variables remain to give a definitive answer and his suggest seems valid.
You could also consider 3 way Xfire of HD 45850x2's or HD 4890's.
Perhaps quad Xfire HD 4870's? Obviously you get diminished returns as you add multiple cards.
The HD 5890 and/or HD 5870X2 is expected to release this year for within the $650 allotment.