I got Two XFX 5870 xxx-edition in crossfire right now and watercooling with a 750w Corsair PSU. So i think u should be alright. Only current draw from watercooling is the electric pump (which hardy uses any power). I forgot watercooling requires more fans in addition to case fans...even with 6 fans my psu still holding up..
I also got 5 hard drives lol. dont have any power issues at the moment.
But i was actually wondering if I would be alright with my 750w if i added a 9800gt for physx?