Think of 2K server as NT4 server and 2k Adv server as NT4 Enterprise server.
Aside from the hardware up's of enterprise server, 8 procc instead of 4, etc....
Then main reason of getting Adv Server is for clustering.
Let's say you want to cluster Sql, that is have 2 or more sql servers access the same data in real time. You will need to "Cluster" them together.
Only NT4 ent srvr or 2k Adv Server offers this ability (at least on the M$ side of the house).
If you are not looking into clustering then there is little reason to go for Adv. Srvr.
If you were going to get to put more than 4 processers on and some serious ram, then I would have to ask what sort of app are you running that would require more than 4 processors, if you were not going to cluster?
But if your intention is to install Adv Srvr on a dual proc box with a 512 meg of ram just to be a web server, then I would say don't bother with it, stick with 2k srvr.