realizing this is literally the first result I got by googling....here's this
http://www.maximumpc.com/article/featur ... its_limits
that looks like a decent article to me.
Personally, I live in AMD land at the moment, and I'm using a combination of weird automated bios overclocking, normal .....manual, hardware overclocking, and software overclocking, which is how I get my 6-core processor up to 4.5GHz
So far, it hasn't gone all Chernobyl all over the place, so....it seems to be ok (In fact, the core temperatures don't seem to go above 38C but I digress)
However, I think you're right to want to do BIOS-only over clocking, I think the use of software overclocking generally leads to unpredictable results and instability. In my case, I'm just using my computer as a Q3map2 compiler, so as long as it's stable for like...3 days, that's good enough. I assume you'll want more longevity and usability.
Make sure your heatsink is good enough for the heat envelope your processor will be creating. I'd suggest the Cooler Master V8 if you have room in your case (but I didn't, and that didn't stop me from using it, it's a great cooler) but if you don't want to carve holes in your case, try the V6, I've heard good things.
Also, disregard the thermal paste they give you, spend the few extra bucks to get diamond dust thermal stuff (for me, about 14 bucks) that really, really helps your computer cool down really really fast.
Good luck.