What you suggest cannot be done. You can't just swap an XP for an MP, just like you can't swap a Xeon for a P3, or an Itanium for a P4. They aren't compatible, for one. Two, they need to be run in pairs. Three, the only benefit is easily seen in encoding software, such as DivX. Most software isn't made to take advantage of dual processors.
I'll try to explain this. In a dual cpu system, the motherboard delegates programs to each processor. So, if you are running a single app, it will be run on one processor, with a few minor processes possibly being delegated to the second. However, if you had multiple unique processes running (ie: winamp, IE, SETI, and maybe DivX), then the processes are sent to each CPU independently, reducing the overall load. That's why dual cpu's are used for servers that are running huge numbers of tasks simultaneously: because a single processor, even if it was the same speed as the sum of the multi-processor config, would run slower. The multiple processors can work in parallel on their own little tasks while a single CPU is relatively serial in nature. So, from what you say you want a set-up would take advantage of this, but you would then have to buy a new motherboard and a second CPU, turning your $100 replacement into a full $500 upgrade. Frankly, at least as I saw it, a single CPU is far superior to dual. Dual is just for show. You'd only have at most three 32-bit PCI slots to put your NICs into, because all dual processor boards feature 64-bit slots. Spend the saved money on some faster ram, and maybe upgrade to a slightly faster CPU while you're at it.
I'm writing this pretty late at night, and I'm tired, so if it's unclear, PM me and I'll re-write it as best as I can.
XP 2500+ Barton
A7N8X Dlx
2x512MB Corsair PC3200
MSI GF3 Ti500 w/64MB DDR
16x DVD-ROM
2x80GB 7200RPM Maxtor
Onboard audio
My computer is bigger than your computer....