OSX Server has been out for a couple years already. (at least it seems that long!) The new version that just came out is the first with the Aqua interface (prior versions looked like a hybrid of OS8 and NeXT) but I dont' know what else has been added.
Basically, if you need a server and want to spend $500 or thereabouts, get OSX Server, if you dont, wait for OSX Client.
g4 400 AGP - 512 RAM
10GB internal (dumping ground) + 60GB (30 GB OSX partition/30 GB OSX Server partition) - 3GB external SCSI (OS9/and I think the Public Beta is still on there somewhere) - DVD-RAM - ZIP