The reference GeForce 6600 GT AGP at default clock speeds is doing 500 MHz. Its DDR memory is (2x)450, thus 900 MHz. Overclocked it was capable of running at 553 MHz core and 1100 MHz memory frequency.
That high 525+ core frequency is something we observed with all 6600 cards that we have tested to this date, it's just amazing. The non GT's that are running at ~300 MHz at default remain the best overclockers though, as you can gain a 200+ MHz boost. The GT is already pushed towards its theoretical maximum clock frequency.
But to be able to use that high core clock efficiently you need more memory bandwidth and memory-wise the overclock was very good. Overall a nice overclock that will boost the framerates a little higher. Take a good look at the numbers in the benchmarks as you'll see a very nice difference when we enable the overclock.
One small reminder though, our overclocking results are never a guarantee for your results. Manufacturers' choices in components differ and so will the end-results. This however is a good indication of what is possible (or not).