the CPU and/or case
the 2800 is not much of an upgrade over the 2200, so the 3000 or 3200 should be ok
if you like cases with Windows in them, the Thermaltake Lanfire case - the least expensive of the Thermaltake cases, but IMO the better looking one, with an X window. the only thing is they don't come with a power supply, but you can still use your PSU that you already have, or if you want you can get a new one
just suggesting