While Xbox 360s have become increasingly capable at handling their crippling heat defects, you still want the newest, coolest 360 you can buy. Right now, that's the “Jasper” edition, featuring a new 65nm GPU. You should note, the GPU appears to be Jasper's major relevant revision, and it keeps the same huge heatsink found on the previous iteration—which is probably a good thing, actually. The other revision is that on-board memory stores NXE in the Jasper Xbox 360 Arcade. (It's unconfirmed as to whether or not such is true in the 60GB models of the 360.)
To pick up a Jasper of your own, look through the hole in the Xbox 360 box. You need to spot the number of amps used by the 12 volt rail—not that hard, we promise. You can spot that right next to the 12V delineation. Since the new system uses 12 amps, look for this number to read "12."
In other words, buy 12V-12 and everything is good. Buy 12V-14 and you are getting an earlier model.