XP 32 bit is licence limited to 4GB - around 3.5GB max after memory space is allocated etc. (it's a licence issue why it's given the 4GB max rather than a 32 bit issue, it can be made to see more but that's another matter...)

Your current system is probably DDR2 memory, an i5/i7 board would be DDR3 so wouldn't be compatible with a later build but adding another GB should be relatively cost effective and will probably give you noticeable results.

Ultimately, an i5/i7 would take you potentially leaps and bounds ahead of that PIV but you're looking at a new motherboard, CPU and RAM at the least for that. If your current motherboard is AGP video then you're also looking at a new PCIE graphics card.

Glad to help, best wishes.