1, Any one are fine. I would suggest one with the dual bios which will limit you to 2Gb models. That's mostly going to be the original reference designs. The MSI Twin Frozr III also has dual BIOS. The Asus 2Gb Direct CuII does too, but it's not listed on Newegg anymore. Possibly it's been discontinued. I don't know for certain.
2, As long as you don't exceed the 1Gb frame buffer the 2Gb has no performance advantage. Unlocking will be trickier though without the bios switch, which none of them have (AFAIK).
3, Should be, but it depends on the rest of the system and if you O/C you CPU or not. A 650W would be a better choice if you are buying a new PSU.