If you have dialup, you should probably choose a project that caches WUs, unless you want a bit of a hassle (something I'm willing to put up with, but many don't have the time or patience). Otherwise, I'd like to put in a plug for my personal favorite:
Folding@Home. It
doesn't affect system performance, so it's a good choice from that perspective. It tends to be more efficient on Athlons, but if you use the "-advmethods" switch when starting the client, it will try to use a special core that works better on the P4, if any WUs are available for that core. Any CPU that is 400MHz or greater can help, but it must be at least at that speed to meet the WU deadlines.
Here are some more links for you if you are interested.