I am going to upgrade my GTS 250 and PSU in September and I was wondering which would be more powerful having 2 GTX 460 SLI'd or one GTX 480, just because both would cost me £400 and would a Corsair 850Watt PSU be enough even if I wanted to SLI 2 GTX 480's or would 1000Watts be better.
SLI 460 768mb models outperform a single 480 until one starts venturing into higher resolutions/AA/AF/tessellation. THe 480 seems to be more robust in those fields, even though at lower settings the SLI460s can mark substantial gains. I haven't yet seen a review of SLI4601gb models. Since there is more differences between the two models than just memory, one could probably safely assume SLI 4601gb would build upon those differences, and perhaps overcome the 480 entirely.
For SLI480s, 1000w > 850w. Theoretically, two 480s could run on an 850w, but would push it to its limits. A 1000w would give a bit of headroom, thats for sure.