upgrade NT4 to W2K – I’m lost

We’re a small company with less than 20 workstations. Our new parent company is in another state and so I’ve been asked to upgrade my NT4 to W2K. The problem I have is in looking around, I get lost in which of the versions to buy. there’s everything from server versions to pro versions to advanced versions. Pricing from $400 to $3500. Some come with CALs, some don’t. I’m getting dizzy here, anyone got a road map through all of this?