Your probably fine with two GTXs if its a newer version of the PSU but im not sure on how to tell.
Read Neweggs Reviews
Issue
Antecs response
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Antec?s TruePower Quattro power supplies, the TPQ-850 and TPQ-1000 have more than enough power to run an 8800 graphics card and are SLI certified by NVIDIA. The actual issue with the graphics card and System Sentinel is a communication issue. The Supply has plenty of power, but the card is not handling it properly.
This problem is not only with the Antec supply. Other brands of power supply are having these issues as well. There have been reports of power supplies from Corsair, ThermalTake, Enermax, Rosewill, OCZ, XCLIO, and Silverstone not working with these cards as well. I cannot post the links to support all of that, but you can do your own search of " 8800 sentinel PSU " to find them.
Nvidia SLI certified our power supply to work with the cards. Months after the launch of their card we started to have problems with the 8800GTX and the Quattro supplies. We implemented a fix at this time because we were the only company with the issue. This fix worked for any user having the 8800GTX issue.
Then Nvidia released the (G92) core series of graphics cards GTS/GT. These cards sometimes do not work with either the original or fixed version of our power supply. We have been researching the issue, and Nvidia has been testing this issue in their labs. The problem appears to be a hardware interaction. With the right (wrong?) motherboard/graphics card/PSU, this issue occurs. Many combinations work, however (we?ve got an off-the-shelf Crosshair+8800GT 512+Quattro 850 system running perfectly in our lab), so changing any of the components could possibly make the system work. It seems that a bios fix/flash is going to be the final solution, but we don?t have any definite answers at this time.
I can get you more info if you like.