One of my slower computers is beginning to lag behind. It has a 550MHz Pentium III with 256 MB of memory. 550MHZ is the highest speed the motherboard will accept. I use it for Office XP, a bit of Photoshop and some gaming. Would adding a second CPU be a good idea, or would I be better off adding another 256MB of memory? Would either make a significant difference?
I suppose you have a dual mobo, because if not, install another CPU is imposible. But even if you have it, I think you won't benefit too much from it. The reason: as far as I know, Office, games and alike don't take advantage of dual CPU.
Windows XP is known to be a RAM eater, but 256Mb is a decend amount. 256Mb may help, but not sure it's worth. Try if some friend can give you some just for testing purpouses.
Another option is try to overclock the CPU and/or the system. Do you have the option to increase the FSB? This combined with good cooling can help and cost nothing to you. If you are interested in overclocking and don't know how to, post you entire system as detailed as you can and people here will help you.