I'm in this boat right now...
I have a self built PC, it was 2.4Ghz C2D on an Intel Mobo with 4GB of RAM, with a GTS 250 with 1GB of RAM. Being dated as it was I dropped in a quad core Xeon (socket compatible) at 2.83Ghz and 8GB of RAM.
After kicking around in Ubuntu & Mint for a while, (running windows in a VM), I wanted that 'nix feel but with native popular program compatibility, so I bought a copy of Lion and I've been off to the races, happy with the UI/UX for the most part. Extremely happy with the performance and platform compatibility.
Now I'm in between do I want to buy a new MP if they ever get announced/released or do I want to a build a workstation class equivalent (with a dual Xeon board, ECC memory, etc), in which case I may as well buy a MP.
Aside from the back and forth going in this thread, the one point of anxiety I have about my hack is OS upgradeability.
I think I may buy a "bare" MP, if a 2012 model ever gets announced and just pimp out the memory, video (if hopefully supported) and storage on my own.
This thread has not alleviated any of my confusion![]()
It wasn't difficult to go from 10.6 to 10.7..but it wasn't as easy as on my Mac's..