Inhibitor and key replacement

Hey all,
I'm looking into the dark magic of "chipped keys" and inhibitors.
Tomorrow, I pick up what I think is the correct lock-cylinder for my Gen3.
(The old one just got sloppy and we're tired of playing with it to get the key to turn)

When "they" say you need to program the key:
1. Does anything actually change in key?
2. Are you just programming the ECM to accept this key?
3. Is there a fixed number of "slots" in the ECM for extra/replacement keys?

Surely, someone out there knows the gritty details...

thanks!

Gory details (if anyone is still reading ;-) )
Current status--------
We had a new key cut at the dealer based on the VIN
Works the same as the old key (will eventually turn with some jiggling)

Trying to reduce the $600 estimate from the dealer, we:
1. Found we can program the key ourselves (after some digging)
2. Verified this worked on the key we just bought.
3. Found the OEM part online (for a 55% discount from MSRP)
(did anyone else get in on the Stocking Stuffer extra 25% discount?)
I pick the parts up at the dealership

So... if the part is correct, my remaining concern is that "I've heard" there are only four slots for new keys in the ECM. Did the two original take up two or just one???