Although it might run, I wouldn't recommend it. For that system you are probably talking in the region of a 500W psu:
This is reportedly one of the best at that range, without parting with alot of cash:http://www.overclockers.co.uk/showproduct.php?prodid=CA-053-BQ
It now runs on a 250W PSU and it draws much less than that; therefore adding a low-power video card can't require a 500W PSU.
Xttony's recommendations make sense.