I would try the computer with only the pc 6400 ram in it . 2 gig is plenty for XP and will run faster than vista with 4 gig .
If the pc 6400 is 2 x1 gig sticks make sure they are in dual channel .
Other than that I wouldnt upgrade components . Save the money for a time when you can make a complete swap of cpu/mb/ram/gpu.
If you upgrade anything then maybe the case , keeping an eye on the next generation of hardware you will install in it