Here in the UK during the winter my x2 3800 idles at 24/25*C, room temperature at 19*C (arctic freezer 64 pro + AS5) It's now summer time and it idles at 32*C, room temp at 25-29*C.
Go for the X2 3800, they are overclocking like mad right now, mine does 2.7GHz with 1.47v and that's on air, with better cooling I recon 2.9GHz is possible.
upto 1.5v on air for 24/7 use, 1.55-1.6v if you're using water cooling. The max I've given mine on air is 1.6v but that was just for 10min while I was benching.
I've got a arctic freezer 64 pro and it keeps mine at 28*C idle and 38*C load (2.45GHz)