Don’t waste your time. You would have to reinstall Windows XP from scratch to switch to a multiprocessor kernel. Better off buying a refurbished Windows 7 machine. TigerDirect has a nice Optiplex 755 refurbished with 4GB RAM, 500GB hard disk, and Windows 7 Pro 32-bit for $250.
I just read through all the comments, but didn’t notice anyone else mention the kernel problem. If WinXP is installed on a single-core CPU it will set the kernel to single-core. If you then add cores by changing the CPU it will very likely no longer boot up at all; if it DOES boot it will still be using the single processor kernel. I’d be really surprised if it boots at all.
Now, changing your XP kernel to multiprocessor is not anything as simple as just swapping out a physical CPU. The kernel mode is set during setup.exe - I don’t know of a way to change the kernel mode after Windows has been installed. If there is a way to do such a thing it is going to involve command shell programming that I suspect is beyond your skill level based on the topic of this post.
Do yourself a favor and just get another machine. Even a refurbished one is going to be much better than what you have now, and the amount of time and hassle you will save will be worth every penny. Add to that the fact the WinXP is end-of-life in two months and the decision is a simple one.