Unless you're using more than 2 cpu's going above the standard edition is a waste of space on the harddisk.
So lets keep the comparison between 2000 and 2003. With 2003 server you'll get a lot faster system already locked down from the beginning, whereas 2000 is litereally an open barn that needs a million patches from the beginning.
You'll see the most immediate improvements to the ADS handling and network utilieis. These have grown quite a bit, and finally their DNS server is really great.
But seriously though, you wont get anything out of 2003 server if you're just into games or graphics.... its primarily a server edition, and hence the server side has been improveed over 2000.
Cheers, Flix