Without knowing the motherboards maker, you should be able to go to 2.8ghz, possibly 3.0ghz.
More RAM is always good. Anything above 2gig is probably overkill with XP.
I would also think about swapping to a larger hard drive. Anything from 250gig on up is good.
If you buy a retail hard drive, it will come with a disk copy utility.
Good luck!