From what I recall, a CPU change counts as 2 points out of the 10 for a change. RAM counts as 0 and the video card is a 1. An identical replacement I'm unsure of, but it may not add points. If you reach the full 10, XP cannot be installed. This, however, only applies to the OEM version of XP. If you have the retail version, you can make all the hardware changes you want. Search Google for this points system if you're interested to know more if you have the OEM version (always the case if you bought your computer off the shelf rather than purchasing the OS separately).
For your reference, I have the retail version of XP Pro and since I bought it back in February of 2006 (just before Vista was released), I've swapped out 2 motherboards, 2 CPUs,, 1 video card, 2 sets of RAM, an audio card, added another hard drive, and upgraded my CD/DVD drive once or twice and I still have no problems installing XP. Another hardware upgrade is in store very soon - another motherboard, another CPU, and another set of RAM all in one and Windows XP shouldn't have problems installing (just make sure the hardware supports XP first though and almost everything does).