Wow, interesting thread.
Pictures will follow -- right now I rearranged my den to house my new Mac Pro so it's a disaster area. I'll take pics when it's finished. Let me describe my current setup:
Main work area:
Mac Pro 3.0 GHz Quad
Dell Dimension XPS5 (Windows Media Center 2005)
Gateway P4 2.5 GHz (Fedora Core 5)
MacBook Pro
Thinkpad X60
Around the house:
Thinkpad X41 (XP -- Media Server and download mule)
Mac Mini (G4) -- my wife's surfing computer
HP Pavillion 3.2 GHz P4 laptop -- Media Center on my 50" DLP TV in living room
HP 3.0 GHz P4 -- Home Theater PC on my 61" DLP TV in media room
Compaq 1GHz P3 -- MAME PC with arcade joystick and rotated 21" CRT
Now, my abridged complete computer history (some ommitted and some collected after their prime are omitted) -- only ones I seriously used are listed. In order, * = still have it:
Atari 600XL *
Atari 800 *
Atari 130XE *
Atari 520ST *
IBM PC XT clone (4.77/8 MHz 8086, DOS 3.2, still have the DOS box)
IBM PS/2 Model 70 (16/20 MHz 80386) *
Gateway 486/66
??? Pentium 133 MHz
Home-made dual Pentium II 266 MHz
Dell Pentium III 800 MHz
Home made Pentium 4 1.2 GHz
I left some out and didn't mention laptops, or the current lineup. And I'm sure I'm forgetting a few

I generally get the upgrade itch when power doubles -- roughly 18-36 months.