Yes. it will work. Make sure you get a z68 chipset motherboard, they're probably your best bet for future compatibility with Ivy Bridge.
You probably will not be able to run 2 550 TI in SLi with a 500watt PSU (at least I wouldn't.) But yes, for just one, thats plenty.
I'd consider going higher clock speed than 1333mhz ram, it wouldn't cost much more (8 bucks or so) for comparable 1600mhz rated RAM. The reason for this is Ivy Bridge is reportedly going to officially support RAM speeds up to 2133mhz.
Yes 650 would be overkill. Although, your computer is only going to take what it needs. So for example, if you put a 1000 watt power supply on your system, and it only needs 230 watts, its not going to hurt anything.
Generally, when selecting a power supply, you want to figure out how many watts your computer needs and increase that by a factor of 30 percent. 500 watts meets that requirement.