In short:
Main: Athlon64 3500+ Winchester (with Zalman CNPS7000B-Cu), 2048 MB Corsair RAM (4*512 MB, 2*200 MHz, 2-2-2-5), eVGA 7800GS 256 MB, 200+120+200+320 GB WD HDDs (on MB and Promise Ultra133 TX2), SB Audigy 2 ZS, DVD-ROM + DVD+/-RW, CM Stacker Chassi Silver, Zalman ZM400A-APF (400 W PSU), Zalman ZM-MFC1
Using XP Pro.
Will probably add another harddrive soon.

Firewall: Pentium 133 MHz, 32 MB RAM, 512 MB CF-card as harddrive, Intel 10/100 and DLink DFE-530TX. Running NetBSD 2.1 with ipfilter and snort for transparent (layer 2) firewalling

Server: AMD T-Bird C 1400 MHz, 1024 MB 133x2 MHz RAM, some small harddrive and a DLink DFE-530TX. Used for running test-versions of game- and portal-server, which include running an sql-server, and also used for some offline rendering.
It's one half of a server-setup, with the other half almost identical (a bit less RAM) and used for some random stuff + calculations. Both are looking for aliens when they're not doing anything else.