I recently purchased an ATI 5570 for my HTPC. I won't be using it much for gaming, the rig in my sig handles gaming duty, but for the hell of it I decided to see how well it OC's.
I was really surprised with the little 5570, ATI seems to of left alot on the table. I was able to take the core from 650 to 865 and the memmory from 900 to 1120. Not bad at all, Vantage GPU scores jumped from the 3500's stock to the 4700's when overclocked and temps stayed in the 60's. very nice results and should be enough to take game play from unacceptable to playable in some situations.
The 5570 is a excellent HTPC card for SFF low power applications and I'm very happy with the purchase although the price could of been better.