Do we lose warranty if we overclock to more than the default 600Mhz (for example someone suggested 850Mhz)? I don't want to risk anything, so I thought I'd ask before I do something stupid

I don't put any stock in the warranty, personally...I doubt very seriously whether I'd have any success getting a replacement, the way things have gone. I'd be worried I'd send something back and have it get lost in the mayhem of their warehouse, with no record of receiving it. I plan to just overclock them, keep them cool and use them as long as they'll last.