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!
What bout a NVidia Geforce 7300? It's not listed on that page.
Also, what kind of stats would a Pentium D 2.8ghz get?