Hi, I currently own an iMac that I want to sell to my sister. I was thinking of buying a mac pro because I need the extra processor power and the easy upgradeability. My question is, just how upgradeable is it? I know that you can add ram and hard drives easily, but what is the maximum watt usage etc? For example imagine a new video card came out like a 1 gb dual GPU ati card usuable in macs. Will I be able to install it ?