Yes the power consumption would make a difference, a HDD consumes about 20 watts when its working flat out, which it wouldn't be doing. But I think the cost of constantly replacing a USB stick would be greater than the electricity cost of a HDD.
Depends on the OS - if you're using Vista it should be fine I'd think - if it's XP you probably won't get much better than with 2GPUs as under XP it generally uses a whole core of the CPU