I would imagine a x3/x4 core Phenom would make a huge difference, but I will leave someone with my experience of these processors to advise you. The link above details every supported CPU though, so should come in handy for bargain hunting.
Essentially, the faster the processor speed (in GHz) and the higher the number of cores, the better. Aim to get at least a triple or quad core (X3/X4 CPUs near bottolm of the list.