Hi So I currently own an early 2008 iMac, 24.GHz 20" etc. I am looking at getting a Mac Pro. But how upgradable are Mac Pros? I know you can change RAM, grpahics cards, HDDs and stuff. But can you do processors and more? Also, compared to my iMac, if I bought a current Mac Pro, how long would it live till it was out of date or become too slow for things roughly? Thanks.