Make 100% sure you're getting a FALCON or JASPER unit. Since it was Summer 08, you won't get Jasper, but make sure you get Falcon at least. The first two boards (Xenon and Zephyr) use the old chips and overheat, causing frequent and common Red Ring of Death (RRoD) errors. These systems have a 203W power brick and on the back say they use 16.5A on the 12V line. DO NOT BUY THESE SYSTEMS, they're almost 100% prone to fail eventually.
The Falcons (third motherboard revision, with HDMI) have a 175W power brick and say they use 14.1A on the 12V line. They use a better CPU that uses less power and makes less heat, reducing RRoD risk, but since they still use the 90nm GPU that uses the same amount of power/makes the same amount of heat as on older ones, they still have the potential to die. The Jaspers replace both the CPU and GPU with less power consuming/less heat producing chips and produce very little heat while running (just got one on Sunday). If you can buy new, definitely try to get yourself a Jasper (November 2008 or newer MFR date), otherwise make sure you get a Falcon.