A 550MHz processor is on a board that can support up to 800MHz with Durons or old AMDXPs. I would get a new system. Here is a link to a really nice system, which you can throw some of your old stuff into as well. Besides, 1.0GHz Durons are like Celerons; aka, they are not good for gaming. They have low L2 Cache, and low clcokspeeds, but their multiplier is set high (13X-14X) It is your decision, but I would not go Duron.
I configured this system for $1,034 with three year warranty, and a 5700 card in it. Check it out.
Or, the AMD64 with a 3000+ with a 5900 for the same price....
Check and compare both. If you want to configure your own, go to- www.cyberpowersystem.com
Good luck either way buddy.