It won't bottleneck it, my nephew plays at the same detail levels I do with the same games on a mildly
oc'd 4600. A 5600+ will offload enough to an 8800GTS to keep it mighty happy. If I can barely tell the difference from my sig rig to a 939 X2 with a 7900GT without running benchmarks I think he will be OK with AM2.
Don't be too hard on AMD, Am2 is still a good unit with plenty of power.