An issue I tell people who buy ATi cards is this: The thermal paste use don the cards soemtimes doesn't melt and allow proper cooling for about a week. I would check those temps, note what they are, and see what others are getting.
And don't regret your purchase of the HD2900 PRo over an 8800GTs. for all you know, the 8800GTS could be having the same problems and you might be saying you'd regret purchasing that instead of the HD2900 PRO. In the grand scheme of things though, it did force you to buy a new PSU, which may sound bad, but hey, at least you won't have to upgrade later on for quite some time. Try to stay optimistic.