Average OC of the 256MB cards is 732/983 according to the hwbot.org database. Which is pretty nice.

It doesn't show shader speeds in the average, but I would say if the core is kicking high speeds it can't hurt, Test with a few different setting with the unlinked setup like was recommended and with a high core and shader, I would personally test all the different setting I could manage to get running.

High core low mem, high shader low core, high shader and core low mem, high all three, high mem/shader low core. Your not specifically using the card for gaming, so its not as big a deal with artifacts (although I haven't overclocked anything past a 6600gt myself)

I'm planning on a new 8800GT but can't decided between the 512 and 1GB, But I won't be buying it for a while, so I might end up with a 9800GT when they get a new core.