I would definitely WAIT for NVidia's next card. NVidia is releasing a high-end 40nm card, quite possibly in the next two months.
Not only does the 40nm mean less power (and noise) right off the bat but the card will definitely use new power management techniques as well. ATI's just released HD4770 is 40nm and very, very impressive for price and power utilization.
Trust me, it's really worth the wait just to have a quieter computer.