I would have two dream machines, one for work use, and one for personal use...

My work box would just be a desktop, support 2+ monitors, LOTS of RAM and a Xeon CPU so I could load Server2012 w/Hyper-V for throwing together VMs to test various things (and one Win7 Pro VM for "normal" work) and the company to pay for a Technet subscription for me.

Considering my current box at work (which so far allows me to do my work) is an older Dell Vostro with a Core2 Quad CPU, 8GB of RAM, and dual monitors, I can't complain too much.

As for a personal "dream machine," something similar to the above, but in a laptop...