Upgrading whole system, upgrade my 8800 GTS?

I'm upgrading my whole system and also getting watercooling.

I bought a 8800 GTS 640mb in march, should I upgrade that again since GPU waterblocks are over 100 bucks, would it be worth it to upgrade my GPU again rather then, get the waterblock for the 8800 then upgrade again next winter and get a new waterblock?

I would go for a 9800gtx+, as they are only $200 and you will see a much larger improvement rather than overclocking your card under water. If you have an intel board, however it might be worth going for HD4850/70 for CF in future.