The recommended power supply for a single GTX 295 is 680 watts. The minimum for a GTX 260 SLI setup is 750 watts.
The XIGMATEK 750W would work well in either situation. If you're willing to fork out the extra cash, having the extra power afforded by the XIGMATEK 1000W wouldn't hurt. Especially if you ever decide to do a GTX 295 SLI setup.