For my in-laws I got
Motherboard: ECS 755-A2 SiS 755 Athlon 64 Skt754 DDR ATX Motherboard w/Audio, LAN, RAID/Serial ATA Retail ***Free 2nd Day***
400/800/1200/1600MT/s, ATA/133, AGP 8X, 2DDR DIMM, 5PCI, 1CNR, USB, Audio, LAN, RAID/SATA $50.99
CPU: AMD Sempron 2600+ 1.6GHz Socket 754 64-Bit Processor Retail ***Free 2nd Day***
w/Fan and Heatsink, 3 Years Manufacturer Warranty $75.90
HD:Samsung SpinPoint P SP0411N 40GB ATA/133 7200RPM Hard Drive w/2MB Buffer ***Free 2nd Day*** $52.00
For you, I would say keep only the external HD, the USB card, the burners, keyboard, monitor and speakers. Compusa Sawmill has a nice case w/ power supply for $30. It depends on what you want to do with the computer. Obviously gaming systems are going to cost more than just an email/online computer. I built theirs for only using AOL and I put Office XP on there in case they need to write any documents.