MAC PRO is the tower version, vastly more expensive 1699 retial minimum. Expandale card wise and ram wise and drives

iMAC 679 minimum no PCI slots... thats the difference. no internal exapnsion except the ram and drive.

I have an iMAC now, 2ghz Core 2Duo... its fast. i use it for production and recording and have no problems with it.

a lot of interfaces are USB or Firewire now anyway, no need for PCI slots.

It really depends if you want to use internal cards or not, with things like TV input cards etc...