At work, I use a five-year-old Apple iMac G4, with 256 MB of RAM and OS X 10.3. It works great. We have a cultural bias against older computers, but there's so much pleasure in working with tools that you know inside and out, and work just like you expect them to.
My work stack's pretty simple. I spend most of my days inside Terminal and Camino (usually RPMozley's optimized G4 builds, for maximum speed). I jump into Firefox while in design mode, TextWrangler while working with text files, GraphicConverter for very occasional image editing, and Psi for XMPP IM. In terms of productivity apps, I use Word and Excel 2004 most of the time, and jump into NeoOffice when dealing with the new 2008 XML Office file formats.
I'm still in love with the flat panel iMac hardware, featuring an elegant 180° swiveling screen I use all the time, turning it so that passers-by or folks on the other side of the office can see what I'm pointing at. It's a shame that Apple doesn't offer anything comparable now.
It would be really easy for me to upgrade, but why? It depresses the hell out of me when I see people I know stuck in eternal upgrade loops, throwing money at unused performance. Or even worse, unusable performance, when all the gains from an upgrade disappear under the weight of increasingly bulky and unusable software, leaving no meaningful boost in productivity.
As a computer professional, and a server-side guy, creativity isn't in the expense or bling of your tools, but how you use them. I'm lucky to be able to work in a space where I can choose good tools, and refine my understanding of them over time. (For that matter, it also helps that we use a lot of web-based and command-line tools, which don't require heavy or platform-specific clients.)
Your computer's probably faster than mine, but I probably like mine better.