understand the reason it didn't preform as well was the modest overclock, it comes with the software to go above that.. but with all overclocking if you go over what they did at the factory it may void your warrenty. what i can say is gigabyte has one of the best reputations of any component maker out there they are the powersupply/memory equivilant of corsair the quality is just almost unmatched and you can bet this card will last for years to come
If you are looking to save money get the Corsair 650 or 750w, 850w is overkill even the 650w can run SLI 460's.
On to the 460s, I would get the ASUS DirectCU or the MSI Cyclone, or wait for the MSI Hawk and see what its retail price is. Those are the top three 460's and excluding the Hawk, they are within in the price range of the Gigabyte you linked.
Luls the PSU your buying is better yet spend on the psu's Helltech mentioned. Since even crossfire 5850 head room would just be a 750 watt PSU. Anywho, another good budget PSU I wanted to mention was the XFX 750 Black Edition or the XFX 650 XXX edition. Plus the 750 and 650 versions are both cheaper than there Corsair opponents XD.
I too will recommend getting a good 750W PSU and use the saved money to go for a better GPU.. What are your other system components.? Depending on the motherboard supporting SLI and/or Crossfire you can go for a GTX 470 or HD 5850.. As for the PSU i too will recommend either the XFX 750W Black Edition or the Corsair HX 750 (whichever is cheaper)..