DON'T GET DUAL CORE YET!! Whatever you do! They run incredibly hot and offer no real-world performance boost, unless you run workstation apps that are SMP aware. I would recommend a nice AMD A64 Venice rig instead
Or, get an old Asus P4C800-E, a Pentium M socket adaptor and a Pentium M 730. I'm going that route soon, Pentium M's beat FX55's in many benchmarks, since they have the shortest pipeline ever to appear on a modern CPU. I think it's ~10 stages or something ridiculously small.