I'm currently running 325/650... I had it at 335/655, but there's no noticeable performance difference... only a temperature difference.
For the record, a Ti4600 runs 300/650 =)
Some people are hitting 680+ on the memory with this card though... although I must question the stability... at 660 I get pretty bad tearing in Dragothic tests in 3DMark2001
I decided to see how much I could get out of it and still complete 3DMark... 350/700... but tearing was horrible... I wouldn't recommend anybody even try these settings, as they're so high it's dangerous to run at that speed for any length of time.