Buying a 4 or 6 year old computer sounds like a VERY bad idea. I do not know much about the techincal sides of hardware - but the architcture of the chips can change so much in that time you can run into problems. Even if you wanted to upgrade it - do mondern components work on a motherboard that was designed more than half a decade ago?
You are also buying a machine that is out of warentee. You could easily have component failure. We try to replace machines every 4 years - sometime we strectch it to 5. We ussually see 1 major failure in that time - ussually HD or Graphics card - sometimes motherboard.
A new Imac should last you 4 years, a 4-6 year old Mac Pro - maybe 2 at best? So it would have to be less than half the price to even consider it. Even then - it seems like more problems than it would be worth.
I am dual platform (Mac by day-job, PC by moonlight) and there is much more value on the PC side. If you can stomach the switch - you can get more power for less if you can exit the cult.