I've always wanted to have a fairly minimal setup.. We don't have a lot of space, and I don't want to waste a lot of energy, so I've always tried to have 1 always on server as my target. I've had two and three at times, but mostly just during upgrades, or while I was waiting to find the time to upgrade..
I have 1 quad core i7 with 32 gigs of ram, and 8 1.5 terrabyte hard drives running in RAID 6. The hardware has two network cards, which allows me to do just about everything I want with virtual machines running under KVM. Things are getting a little tight these days, so I'm looking to upgrade to 3 terrabyte drives for 18 terrabytes usable space, and I'd also like to move up to 64 gigs of ram, but I'm going to wait as long as I can so maybe the prices will come down a bit.
The server runs cool and is pretty quiet as I've chosen to go with a 4U case with the largest fans I could find. I've got a 24 port gigabit switch and an access point to round out my hardware.. Everything else is virtualized.
4U server - 32 gigs ram, i7 9 terabytes usable RAID 6, and dual nics
1U 24port gigabit smart switch
2U rack mount UPS
generic 802.11 b/g/n access point
4x part time windows XP for experiments and various utilities
3x part time windows 7
relatively low power (about 150-175 watts most of the time, and during the winter it just reduces the amount the electric furnace runs)
All your eggs in one basket - I have backups, but I'll be down in the event of hardware failure.
VM host is a little weaker than is optimal when I'm running a lot of guests