You can air-cool a peltier around the 50 watt range with no problems. I have a p-III 700 oc'ed to 933 using a 50 or so watt pelt and an ALPHA cooler running in FLORIDA the land of humidity for over 2 years now.
The temps on the CPU are around 80 degrees, perfect.
If you never setup a pelt before, be ready to kill at least one trying to get it right. It is very easy to burn a peltier out trying to figure out which side is the cold or hot side. For your cold plate use the best material you can find (copper at least) and that goes for any thermal material you use as well.
As for power requirements I had it running with a 350 watt power supply with no problems and the rig uses a CD-ROM, and 2 harddrives, along with about 3 case fans.
Have fun, I did.