A bit more modern than what I'd consider "retro" but this AM2+ PC is certainly old and does serve some classic gaming needs -
I replaced the coin cell battery in my Packard Bell iStart 2380 PC and overclocked it's AMD Sempron 3500+ (stock: 2.0GHz) to 2.7GHz and underclocked the RAM to 533MHz. I had the CPU at 2.9GHz at one point, but it wasn't stable.
Rest of spec is as follows:
Biostar MCP6PB M2+ Motherboard
2GB DDR2 RAM
Nvidia GeForce 7300LE 128MB PCIe Graphics Card
200GB SATA HDD
OS: Mac OS X 10.6 Snow Leopard
This setup is mostly useful for the addictive "Marble Blast Gold", but I've also been playing the Mac ports of Halo & Tomb Raider: Angel of Darkness.
This PC is certainly more than a match for my PowerMac G5 Dual 2.0GHz, but it's seriously in need of a replacement Graphics Card, as the 7300 with the custom Hackintosh drivers does not like certain OpenGL games, for instance Tomb Raider Chronicles is just a blank screen. The 7300 was the only OS X-compatible card I had to hand when I set this system up and even that was a nightmare to get up and running.
I know this motherboard can handle much faster processors, but for what I'm using it for an upgrade would probably be overkill.