In that case, basically everything should see a huge improvement.
Oh, and you know you need new RAM too, right?
Yup, Corsair XMS3 1600 , 2 GB installed at the moment. Since I'm running XP 32 bit I've heard installing all 4 can actually slow things down - It's a mixed bag though, some think it does, some claim no difference. I personally think 2 GB should be more than enough with XP anyway.
Edit - I definitely plan to use all 4 once I upgrade to Windows 7 64 bit