Oldest but still running is my HTPC/Media PC. Although, It's about to be replaced with a i3 build.
Athlon 2800+ MSI KT880 Delta nVidia 7800GS 2Gb DDR 400 It was a gift after building a new machine for someone.
Mid-2008 My main pc is a C2D Wolfdale E8200 Asus P5K-E 4Gb OCZ Reapers BFG G92 8800 GTS 1Tb WD Black FALS Though not the latest and greatest out there, it does what I want and need in a timely manner. When it can't, it'll get replaced and re-purposed.