Please don't try to run two 8800GTXs off a 250W Thermaltake PSU. Just get yourself a nice high quality main PSU *cough* jonnyguru.com *cough* and save yourself some hassle.
Also, I read that the 680i had some issues as well, but that was close to 3 months ago, and all I've read lately has been positive. EVGA has a new revision and an updated BIOS. As far as I know, the 590i still doesn't have an fsb strap for the memory, and has always been craptastic in the overclocking department. If you're not into the 680i, just go with 650i or 965 - no sense shooting yourself in the foot.