Unix cluster: 25 Linux PC's; 4 Digital Unix; 4 HP-UX. All of these are on desktops - no dedicated server. Central services (home directories, mail server etc) provided redundantly by one of two PC's. Several of the Alpha and HP machines run as X terminals to Linux boxes (on top of the vendor Unix.)
Other desktop machines: another 10 PC's and 5 Mac's. PC's mostly Windows 95/98 with a couple of user administered NT and Linux boxes. No NT server: SMB services provided by Samba on Digital Unix server pair.
Babar Group recently received a Sun E450 + Raid array which is partially integrated into the cluster for convenience: shared passwd file, home directories etc.
About to finish moving from Alpha "server" pair to Linux PC "server" pair with software RAID-1 for fault tolerance. Don't envisage buying any more vendor Unix machines unless experiments have specific needs (eg the Sun.) Don't envisage any big move to Windows NT/2000.
Security: have a firewall administered by HEP group itself. Block most traffic to random sites (telnet, ftp, X, NFS etc.) Strongly encourage ssh (had good results with putty on Win 95/98/NT.) Don't use NIS for passwd, automount etc anymore.
Trying to simplify complexity of system: hence move to use HP and Digital workstations as X terminals as much as possible, so all users share common Linux interactive environment; also trying to avoid unnecessary daemons: eg rlpr instead of lpr/lpd on client machines.