ATITool is the defacto tool for checking for errors. Looping 3DMark05 or 06 really is not a true stress test. I have many times been able to loop these programs and get some pretty awesome scores and yet it failed ATITool every time.
Realize, many people claim overclocks that are not truly stable. I use www.dfi-street.com
as a good guide for a definitive stable overclock. Still, it is your machine and if you don't care about absolute stability it is your call. I am very careful with my equipment and have overclocked for years. To me a stable overclock is far more important than a max overclock. But again, system stability is very important to me. It might no be so for you.
I do find it funny people really worry so much about max clockspeeds when the difference is so minimal. I used to be the same way. If I could just get this speed or that core clock. Eventually you realize it is not really a very big difference. At best you are talking 1-2 fps maybe. And the amount of time I spent tweaking memory timings to get that extra 100 megahertz or taking apart the vid card for AS5 was so time consuming for me for such little gain. Perhaps for those on the cutting edge, and truly great fps gamers this could indeed be important. For me, I get what I can get stably and move on...