Wow. Sorry to hear you had bad experiences with two machines. I have purchased about 30 over the last eight years. The model that has been the most problematic: the eMac.
The individual machine that has been the biggest lemon I ever bought: my daughter's G5 iMac. The one I purchased for my son that same Christmas has never had a problem, but my daughter's has had two logic boards, a new hd, new sensors, and a new power supply-- all covered by AppleCare and they sent their technicians to me.
Hopefully the third time for you will be the charm, instead of the third strike...