the pci bus isn't meant to be raised (for nVIdia, it's 100 or 101 depending on your card and for ATi it's 105)
if you want to overclock, first learn about it and let your card burn in (burn in=stress it somewhat=play games & run 3dmark 05)
once you get the hang of it, find the best drivers (you shouldn't be using stock ones anyways) and download them, along with coolbits 2 (all can be found on www.guru3d.com)
just detect optimal frequency (3d only, 2d doesn't matter) and run 3dmark05, if it doesn't artifact or BSOD, bump the speeds up a tiny notch
I always just leave my computer on overnight and run 3DMark on it. Then I just run Aquamark 3 the next night. Then I bump up the mhz with Rage 3d Tweak.