Hey dooode, Calm down
I just bought a 3ghz Quad 3 days ago and I'm so damn happy with this machine that I could care less if they come out with a new case tomorrow (they can't change much than that for the next few months). The X1900 might not be the fastest card out there but it's blazing fast. I spent another big chunk of money buying RAM and Hard drives (BT and AE came installed)
This machine has a power supply capable of running my home's AC system; it's built with industrial quality, expandable, Did I say FAST?
The thing is that once you finally buy into a Mac Pro system you won't want Apple to renew it every 4 months like they do with the MBPs.
And, of course you already know it; your new computer will be yesterdays technology very soon after you buy it, so the best thing to do is make enough money with it to be able to change every couple of years and deduce it from taxes; or if you don't work with your machine, than pop a $100 dollar bill into a jar every month so you can upgrade when you feel like it.
I have some top end DSLR cameras that get outdated evry 1.5 years and also a wide format printer with similar fate.
Enjoy the new technology. Let it make you happy.
Bests