yea i would say the 4 pin motherboard power would be a issue for high clocks but i have seen alot of 775 boards that use that push some CPUs pretty far. How long did they last? i have know idea but they dont limit it much. if you get a cpu that will overclock with a minimum voltage bump then i dont see the problem in it.
7970 3GB = 8 Threads
5770 1GB OCed = 2 Threads