Here is the box I'm building for my mom. Its got a crapy video card but she doesn't need a good one. If your doing heavy gaming you want something like a GF 6600GT ~$160.
512-B-SF ATX Mid Tower $55
Microstar (MSI) K8T Neo2-F ATX Motherboard $97.5
AMD Athlon64 3200+ $190
DVD-RW $44
Hitatchi 80GB Hard Drive $56.74
2 512MB DDR400 DIMM $93.56
Chaintech Nvidia GForce FX5200 free (I got a spare)
OS Ubuntu 5.04 (She is even knows her way around.
She already has a monitor, sound system, keyboard and mouse.
$536.8