I have mine at 245/525. Another 5mhz on either and I start getting very faint visual artifacts.
I haven't done 'extensive' stability testing, but I did play Ghost Recon with most details max'd for about 5 hours.. even with my overclock, Ghost Recon still
only gets like 25fps with all the quality/details max'd =)
Unfortunately I might have to exchange it to do a faint wavyness/shimmer that seems to come and go. It's definitely influenced by the rest of the system, but my Kyro II is perfect. Sometimes by turning on or shutting down Rain 2.0, I can see an immediate change in the wavyness.
I'm going to try out my friend's card (same BB deal), and going to wait for my new XP system to be built (this week) before I exchange this fast card. ; ) I wouldn't be surprised if the problem disappeared on the new system.