It's socket AM2, btw.
The fastest chip you can get for your computer is the X2 6400+ with 2x1 MB L2 and a 3.2 GHz clock speed. Yes, it will be faster than the 4600+ by some margin and it's not all that expensive ($220 or so) but I'd wait for the Phenom CPUs to come out to upgrade. They should work in your system just fine and will be a much bigger jump in performance, particularly if you get a quad-core unit. Those are supposed to be in the $200-300 range as well.