I recently found a good review of the 5600 am2 with benchmarks comparing it to the Intel core2 duo cpus. For some games, the performance difference was less than 5%. If your games are running smoothly, I would suggest you wait for nehalem, which will require a different board. Socket 775 will be outdated in less than a year.
I don't think you would notice much difference. For gamming anyways...it's not worth the cost of the upgrade. If your going to upgrade go quad core and make your computing faster, not just your gaming.