I'm upgrading my desktop computer. Its not that old (bought in 2007 or 2008), I went out and bought a LG 27" LCD Widescreen, plus some cheap speakers, beats the stupid pod speaker I had.
So originally I was looking at upgrading ram (from 4gb to 8gb) and a SSD but both are too expensive. Keeping the 4gb ram which is plenty. Will probably just go 350-500GB 7200rpm or 10krpm sata. And partition the HD to a Windows O/S (either 7, 8, or 8.1) and Linux (to give it a try). I believe there's a way to boot from one or the other. If theres a cheap graphics card I will throw that in, beats the onboard one I have, which the benchmarks rate as pretty low.
That Laptop idea is an interesting thought, I would do that in a heartbeat if there was a way to "extract" the new O/S on the laptop and install it on the desktop. I must admit I had that thought myself a couple years ago, but everything is "attached" or "signed" to each other. I would get a new laptop and get rid of my old one with the battery shot, the old laptop is good for music downloads and such, keep the junk off the desktop.