if it's primarily a gaming machine, i'd like to know what GPU it's using, you may be able to save yourself the CPU and mobo upgrade and get a better GPU and still run games much faster that you currently do.
However, if I were to choose a mobo, cpu, RAM combo I'd go with an Nforce4/Nforce3 mobo (socket 939), AMD 64 3200 (Winchester core) and 1 GB of PC 3200 RAM; shouldn't cost more than $400