Well, OP, there are two lines of thinking. One is, if you have the itch and have the money, go for it. On the other hand, you may want to ask yourself, do you really need 3870? I'm in a sort of similar position (loosely speaking). I just got ECS 8800GT with S1 cooler installed for cheap, and now I'm hearing that 9800GTX/GT is just around the corner. 9800GT/GTX might turn out to be much faster than my existing 8800GT, but the thing is my 8800GT runs everything I throw at it at a very decent framerate, runs cool and unless I'm gaming I always run it passive. So even supposing 9800GT/GTX will be significantly faster I won't be getting it because I'd be spending money for no visible gain (I won't notice any improvements in framerate or visual quality). So untill such a game comes along that actually requires me to upgrade my videocard, I won't even consider 9800. I bet someone will inevitably mention Crysis, well my 8800GT runs Crysis very well with everything on high except shadows which are on medium. Sure, I may not be getting 60fps with it, but it's not laggy either and I won't be upgrading my videocard just for one game.
So op ask yourself, do you really need 3870 or are you satisfied with 2900PRO, which I think is pretty close performance wise, especially overclocked to 3870. Power usage is a valid concern, 3870 will run cooler, quiter and will not heat up your case as much, so if those are important for you, then you have a valid reason to switch, but visually you won't be getting THAT much performance from switch.