minifridge1138
macrumors 65816
Well, you can't "officially" upgrade the processors in a Mac Pro. Unofficially, yes (and it isn't that hard).
Apple just won't cover problems caused by an unsupported upgrade under warranty. Therefore, an Apple Store employee is going to tell you "no".
Absolutely correct.
Apple does not consider the CPU to be a user-serviceable part. It is also not a part that they will upgrade for you at the genius bar. So the official word from Apple is "no".
As for which Mac Pro is best for you, only you can answer that. Look at what you do on a daily basis and try it on both machines.
As for ram being a bottleneck, take a look at the activity monitor and see if you're using any swap space. If you're not, then you don't need any more RAM.