Here's the best way to find out. http://extreme.outervision.com/PSUEngine
Based on my playing with that Power Supply Calculator, you should be able to get away with a 6850 or GTX 460 with your existing power supply. Assuming it's not too old (as they degrade over the years).
The GTX 460 recommended 322W and the ATI 6850 recommended 290W according to that calculator.