You system is well-balanced; I'd leave it as it is.
If you must upgrade something, you might consider adding RAM. Assuming the @GB RAM is really 2GB: 2 sticks x 1GB, you could add 2 more sticks of 1 GB Kingston Hyperx @GB PC2-6400 DDR2 800MHz CL4 (4-4-4-12) SDRAM DIMM, Non -ECC .
-- If your XP is 32-bit (x86), the operating system will not be able to see or use more than ~3.5 GB of memory, so you be buying 1 GB you cannot use.
-- If you have XPx64, you could use all of the memory. To check your version, right-click on My Computer, and select Properties - that will show the Windows version.
-- If you add RAM, you may have to increase the norhtbridge voltage by one or two increments from the default in the BIOS - 4 sticks increases the northbridge load.