I have a Gigabyte GV-N66256DP (256MB AGP GeForce 6600 with the built-in Zalman heatpipe cooler). The card has a molex power socket, but if I plug it in, the computer won't POST. Oddly enough, it will boot without complaint _without_ the molex plugged in--but I get hard locks and THREAD_STUCK_IN_DEVICE_DRIVER BSODs (weirdly enough, usually just while sitting at the desktop--I've run 3DMark05 through 5 times and played some BF and MOH:PA without any issues, which is why I don't think it's a heat problem...). My PSU is a 420W Thermaltake, and it works fine with a 5900 card, so I don't think it's the problem.
I'm thinking I just got a faulty video card, but I thought I'd ask around to see if anyone else had heard of this issue.
FWIW: the rest of the system is an Athlon 64 3000+ in an ASUS K8N-E Deluxe with 1GB (2x512MB) Corsair memory, running Windows XP.
Also note: Gigabyte clocks the GPU at 400MHz, faster than the 300MHz of typical 6600 cards, which is presumably why the non-GT has a molex connector in the first place. Oh, and it didn't seem to clock it down for lack of external power--see http://service.futuremark.com/compare?3dm05=1815569