This is a good place to look for stats on GPUs: http://pastebin.com/AvymGnMJ. A 4650 should expect about 33Mhash/s. In slush's pool, 1Mhash/s is about 1 share per hour, and each share is roughly .002BTC. So, a conservative estimate of 30Mhash/s would put you at mining 1.44BTC/day. Hopefully that will cover the cost of the electricity you will use mining that!