Good SPU for the gtx 650ti

Any quality 500-550 will run it like nothing(with plenty of power left over). I think the Nvidia recommendation is 400-450. Remember, look for high 12 volt current when selecting a power supply.

Personally I KNOW you can get away with less if you have a lower powered system.

If you can swing a rebate.
45amps @ 12 volts(540 watts @ 12 volts). I would bet this has 2x the power you need for a full system :)
http://www.ncix.com/products/?sku=62824&vpn=CP-9020001-NA&manufacture=Corsair&promoid=1312

Something a bit cheaper. As long as you are not overclocking to the moon, this should work as well.
32amps @ 12 volts (384 watts @ 12 volts)
http://www.ncix.com/products/?sku=76034&vpn=CP-9020046-US&manufacture=Corsair&promoid=1312