Number of passes is less important than the problem size in my opinion... I can run 2GB problems all day at 1.4v (LLC off) but 5GB problem sizes will always produce a BSoD in less than 10 minutes, go figure. 20 passes at 5GB takes around an hour and twenty minutes though.

Currently my setup is averaging anywhere between 53 and 55 Gflops. The more stuff fighting for memory space the lower the number will be.