think its cos some/most o/c software that works in windows sucks. it can also get messy if you mess up.
personally, used clockgen and it worked great in the end. first thing i did of course was slide from 2gig to 2.5 gig straight away (10x250). PC rebooted itself instantly and my windows install was a goner! god bless ghost!
took it a little easier, with small voltage bumps and leaving the ram on a divider. used clockgen with superPI and occt to test the overclock. only rebooted to save the settings properly in the bios once each stage had passed (memtest,occt, superPI,prime and s&m)
at the very end i then worried about the ram