I ran across this fairly extensive article doing various tests of CUDA based cards for PPro CS5.
Do yourself a favor, read it before posting questions. http://www.studio1productions.com/Articles/PremiereCS5.htm (I hope Gary doesn't mind where it links to.)
It covers CUDA cores, memory types, bit rate, clock speed, power supply needs, On-board cooling fan design, discusses end results of render times for videos, and unlocking all unofficially compatible CUDA cards.
Be sure to read the section called "Number 10"
Cliff notes for the lazy: Don't buy a card with less than 96 CUDA cores and less than 1GB ram DDR5 memory. Lastly, make sure your power supply is powerful enough for your pick.
Based on the article I personally bought the GTS 450 and unlocked CS5 to recognized it.