As the others have already stated, a 4 cabinet PC build is going to be VERY expensive.
If you have the cash for it then go all out.
But what I have done is build one nice rig, decent specs,audio, g27 etc - youtube.com/aletchet
Then I am currently in the planning stage on buying a cheap seat + steering wheel set off ebay at a deal (sub £30)
A cheaper, still half decent wheel like the Logitech Driving Force GT (good second hand or new) as these are quite cheap wheels, say sub £50.
Then use some old gear (core2 duo) + okay graphics card - for model2 games, might be able to play some of the older NFS games etc. as the Second rig of multiplayer.
Hopefully the second rig wont cost more than about £150 in total, well that's the idea
- Then you can play some multiplayer sega rally / daytona usa etc as the very least . Then in a few years i'll probably upgrade my main rig and use that for the second player.
If you have the cash I would get a pc + ps3, then use a usb port switcher device so you don't have to unplug anything. HDMI to the TV/monitor for the PS3 and then DVI for the PC.
I have the SNES, N64, Game cube and wii emulator working on my rig, so I can play all the mario karts