Well, I have a 275W Silencer, PC Power & Cooling PS. Their 300W Turbo Cool model is AMD approved, so I figured this one could handle it too. Nope. Tried to use it for my new 1.2GHz fully loaded system (1 HD, 2 CD/CD-R, 256MB RAM, 3 cooling fans (2x80mm, 1x120mm), GeForce DDR, SBLive, NIC, modem).... It was a NO GO. System booted fine, but not stable during Sandra memory benches or 3DMark, and during CD installs of games (like UT) my system would reboot.
Better watch it if you plan on getting a 1.2GHz+ TBird with a GeForce, cause those 2 together suck up some massive wattage.
Shouldn't damage anything though. Worst you should experience is random lockups, reboots, or your system failing to boot.