The limitations are not the CPU's, but rather the board, since they are locked multiplier CPU's. The only overclocking of these boards are of the BCLK, which on average most boards cant get much past 200, which gives a CPU Speed of 3.2GHZ. It could possibly go further however I never went past the 200 BCLK because I was quite pleased with the results. See the results here :
wPrime 32m Benchmark Competition and
wPrime 1024m Benchmark Competition
Plenty of CPU power!!