Multiple GPUs

I've never understood how a multi-GPU setup works.  I got a decent motherboard a little over a year ago, and it has just one PCIe 16x slot, which I thought was typical.  I have an 8800GTS in there.

If I were to get a second card, would it work in one of the 1x slots?  Or would I need another 16x slot/ some other specific hardware configuration?

depends on the card, but yeah in general yes you need the same ports and you need the same model of card (preferably, might be hard otherwise)

You want to wikipedia ATI and nVidia on crossfire and SLI.
Not sure which is which.