I previously had a BFG GTX275 896MB OC. The 460s are much shorter - approximately 3 inches. I got two eVGA GTX460s 768MB OC. They are in an SLI configuration. So... I really can't speak with respect to the performance of one.
Otherwise, I am very pleased with my purchase. The cards are really quiet and run relatively cool. I'm in Canada, and the summers still get very hot (40 C = 104 F in some areas, plus high humidity!) I'm close to the mountainous area where it can reach about 35 C - 95 F. Using GPUz, I've only seen the cards get in the mid 60s (Celsius). I'm using an Antec 900 II Gamer case.
I would think if you are buying just one of them, go with the 1GB version. If you are doing an SLI configuration, I'm guessing either of the 768MB or 1GB versions would be sufficient.
If I knew how to (relatively new member), I would post some images of my set up and a side by side length comparison (460 vs 275).
Also, the eVGA warranty is excellent to have.