the only difference between the e6600 and the e6700 is that the e6600 has a multiplier of 9x and the e6700 has a multiplier of 10x.
all this helps out on is overclocking, but when your using multipliers or 9x and 10x, they really don't make that much of a difference unless you have phase change and you are trying to overclock to an insane speed. then there is the clock speed but you can easily overclock the e6600 to e6700 speeds, EASILY. so get the e6600.
here are 7900gtx's on ebay:
http://search.ebay.com/7900gtx_W0QQ_trksidZm37QQfromZR40i love evga also, if you get an 8800gts, get an evga.