I dunno how much CPU power recording and mixing uses but I don't see the point in spending a billion pounds on an i7, and then a 2nd billion on ddr3
the only thing I do that bothers a CPU is play the odd game and a top gaming rig without going silly is about £800 if you build it yourself, and that's more about graphics card power unless your still in 1999 and play games at 800x600
but idle computers still use tons of power and I'm really curious if one of those atom based PCs would pay itself off for browsing without too much of a performance hit through the savings made on the electric bill
Depends what youre recording and mixing. The recieved wisdom is that lots of tracks + lots of VSTs needs lots of power. However, I've run into significant reproduction problems with just 8 guitar tracks and a sinlge midi of moderately dense drums on 7 and 8 minute songs, such that I've had to alter the latency to get rid of popping and crackling while it tries to retrieve all the samples and play the guitar wavs. Thats part soundcard (EMU and then fireface, so no problem with either of those) but significantly affected by the speed of the computer.
Given that, I'd rather use something way too powerfull than something thats not powerfull enough.
Also, one needs to keep ones audio PC clean and clear, so one for that and one for games and internet.