According to this:
Your motherboard supports DDR2 667/533/400 RAM
So for memory I suggest you go for:
2 x 2GB DDR2 PC2-5300/667MHz CL5 1.8V Non-ECC Unbuffered 240 pin
The 667MHz RAM is backwards compatible with the slower 533MHz and 400MHz RAm and is just as cheap (if not cheaper).
But before you splash out on RAM, why don't you open Task Manager and see if your PC is swapping out to disk (much / at all). If the swapping is minimal / non-existent, then upgrading the memory won't achieve anything.
You might find that tidying up your existing software installations (i.e. uninstalling gumf that you don't use / need) will help.
Also, search these forums for other postings where folks have suffered performance probs. There's often some useful advice (esp. from Fruit Bat).