How to connect two PC's to work together?

I hold two PIII-500MHz PC's, can I connect these two PC's to act as single PC beside dual processor. What hardware changes I hold to made and also which OS I should use? Will it deliver good gig?

How to connect two PC's to work together?

This is called clustering and can be expanded to any number of PCs. You probably entail special high speed grating cards and a compatible switch (for just two computers a crossover lattice cable might be enough.) The best OS for this would be Linux and the software for linking the CPUs is call Beowulf. This was true a few years ago. There may be better solutions available today but you can dig out on keywords cluster, linux, beowulf to find out what the latest and greatest systems are. I am sure that near are Windows solutions as well.
you can't take home them act resembling one pc.

if u want to connect them together check if both have LAN cards and buy a LAN cable this thing is call P2P "peer to peer"

*i think the best operating systeb for them is window 2000 or ME
If you still want two computers to work as one with highly developed performance - read some information roughly speaking clusters. this solution mostly used for high-prformance computer systems, and it is also not cheep.
The better way is that you buy a alien and a more powerful PC. Doing that will save you more time and frustrations than trying to connect the two PC'S.