How can I make the most of old CPUs?

I am about to retire a number of PCs at work. I have a half-dozen Compaq desktops, all 1GHz; a similar number of Dell Optiplex GX270 - three at 2.8GHz, two at 2.6GHz - and one 2.8GHz GX280, plus all the memory. Does anyone know of a motherboard that I can use to take a set of these CPUs, either the Compaqs or preferably the Dell, and make either a two or greater processor unit to give some oomph to a home-made media centre/DVR that I shall also be putting other used parts into?

It's all new to me, but before I start buying some bits, I need to know that the essential guts would work first. I’ll be doing the same with a bundle of Xeon processors all out of Poweredge 2650s as well if that would be better, although some at least will stay in their chassis to support the desktop unit if I can build it.