Anyone else had this?  At around ~880 clock, the cards I have been using are pushing around 350Mhash.  When I bump the clock rate up higher, say 900+, the mhash rate goes down (sometimes by nearly half).

6950s are currently not terribly cost effective for the hash rate they produce

whichever you can get? 

5 hours online?

With 5770, you're also going to buy more hardware overall to get the same Mhash