5.5 (latest update) I'll be updating to 6.0 when it releases but will be staying with the same hardware, it's old but more than adequate for my needs.
Hardware:
dual P2 333
512 MEG
3 22 GIG SCSI drives on a Proliant 2/P card, RAID-5
6 domains hosted with web sites
approx 20 external email users, 4 internal network users
ADSL Internet connection, static IP
Numerous contribs: squidguard, sarg, user manager, qmail queue manager, sysinfo, osCommerce, gallery, netjuke, analog - there are more but I don't recall them all right now...
Ive had some problems, most were a result of installing contribs that conflicted or caused problems with the default setup. Not all contribs uninstall nicely, have had problems removing some.
Uptimes are usually less than 30 days, mostly because of ISP problems requiring a reboot to reconnect.