There are still people who would want the 2900XT, although on greatly reduced prices..... =/
Anyway, I think the 3870x2 consumes the most power among video cards, followed by 2900XT and then 8800 Ultra, but then all of them consumes so much power it's nearly irrelevent to compare them that way.
__________________
Current PC specs :
AsRock 945GCM-S
Pentium Dual Core E5200 @ 2912Mhz (233x12.5)
Palit Radeon 4850 Sonic 512MB gDDR3
2x2048MB Kingston DDR2 800Mhz RAM