My last workstation and server both lasted around 5 years - server got a couple of new disks, workstation saw some disk changes, got an SSD, a new SSD when the first (X25-E) fried, and had a single GPU upgrade (GT8800 -> GTX460). Upgraded both during the last few months, and these will hopefully last 5+ years as well. I ended up with:
Workstation: ASUS P8Z77-V PRO
Corsair 16GB DDR3 1600MHz (4x8GB)
Intel Core i7 3770 Ivy Bridge
INTEL SSD 520 Series 120GB, 2012-07-13 efter X24-E fried
(below this line: parts from the old box)
Corsair TX550M
Western Digital 2.5" 300GB VelociRaptor
Gigabyte GeForce GTX460 1024MB @ 2011-02-12
Server: Corsair 8GB DDR3 1333Mhz (2x4GB) XMS3
ASUS P8Z77-M
RaidSonic Icy Box IB-554SSK
Corsair Obsidian Series 550D
be quiet! Straight Power BQT E9-400W
Intel Core i5-3550 Ivy Bridge
Win7-x64 on the workstation, Debian-x64 on the server.