Don't bother overclocking the core for one thing.. GF2 is memory bandwidth limited and that will doubtfully give you even 1 FPS with the added risk of overheating the GPU... = overclocking the core is pointless.
The memory on my particular GTS only likes to be run in the 365 range with the 6NS SGRAM chips... It's pretty bad overclocker but the FPS benefit from that is still half decent.. good luck on your OC!