well there are a few threads over at Xtremesystems about unlocking the locked pipes on the 6200's. But by now i think, most companies will have changed to an official 6200 core, so there might not even be the extra 4 pipes to unlock.
yeah, rivatuner is a good way to overclock graphics cards, but you could just download the coolbits registry patch for the nvidia drivers, then you get an overclocking tab in the nvidia drivers without having to install any programs. just google coolbits to find it.
3dmark01, 03 or 05, aquamark etc, the list goes on, generally for benchmarks people use one of the 3dmark programs, because everyone is familiar with them.