This is just my opinion but, if I were you I would go with server 03 simply because while I am still in college I have taken some Server 2000 classes and some Server 2003 classes. but, now things are starting to migrate over to the server 03 areas. I wouldnt say my server 2000 knowledge is obsolete but, things are heading away from 2000 I think. There isnt really THAT many differences between server 2000 and 03 mostly graphical stuff but, there is some security differences etc... Personally I do like it more than 2000 it just seems to run a little smoother and have more bells and whistles but as for cost im not quite sure. I wouldnt think that its much more expensive to implement. You said you had 20-30 workstations on your network so that is more than a peer-peer network which has a maxium capability of 10 connections. So, knowing that I would suggest going with Server 2003 Standard Edition or Enterprise but, once you get into the Enterprise size that is quite a bit bigger and most likely more expensive.

MS white paper on Server 2000 and Server 2003

goodluck,tyler.