My main home PC I built in 2002 or 2003 (can't really remember!) but it still runs nice and does what it has to...
OS: Windows XP Pro (yes I still use XP on mine!)
CPU: AMD 4800+ X2 OC'ed @ 2.8 Ghz
Memory: 2 GB (2 x 1GB) Corsair XMS @ 2-3-3-6 (dual-channel)
Hard Drive: 150 gig Raptor 10,000rpm HDD
Video Card: ATI Radeon x1900xtx 512mb
Sound Card: Sound Blaster Audigy 2
I went and built my kids a PC a few weeks ago with these specs (runs great, they love it!)..
OS: Microsoft Windows 7 Home Premium 64-bit
CPU: AMD Sempron 140 Sargas 2.7GHz Socket AM3 45W Single-Core Processor
Memory: 2 GB G.SKILL (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300) Dual Channel
Hard Drive: 160GB Western Digital 7200 RPM
Video Card: Onboard ATI chipset
Sound Card: OnBoard HD Realtek Audio
I also have another cheapo PC running my home security system (video cameras and DVR) and a laptop for mobile use.