The 3870 is TONS cheaper, cooler, and less power consuming. It performs on par with the 2900 PRO/XT.
No it does not consume less power
Don't forget the fact that 8800GT performs around 15% better, 8800GT might have less performance per watt than HD3870 at idle but it have much better performance per watt at load
AnandTech: ATI Radeon HD 3870 & 3850: A Return to Competition
And no, HD3870 is not tons cheaper
but on the other hand the 3780 is better and you can put 4 in crossfire with the new amd boards.
Yea but putting 4 cards in crossfire would cost you around $900, and it will not give you 4 times the performance, not even 3