I bought the parts for my last computer on newegg.com.
I could have put together a computer for $150 if I had wanted. They’ve got good deals, etc…
The most important thing is choosing a motherboard. Do you want AGP (cheaper) or PCI-E? Also make sure it has enough USB, Firewire, etc ports. Decide if you want audio/vidio on the motherboard (see below). Also make sure the mobo can accomodate the desired number of drives, and amount of memory. It should have SCSI, SATA, and/or ATA bus slots, depending on the drive you buy (see below).
Choose the cheapest graphics card that fits your needs. If you don’t do gaming, and don’t do graphics programming, likely the bottom of the line NVIDIA or ATI card will work great.
If you have an AGP motherboard, the graphics card will need to fit into the AGP slot. PCI-E mobo, you need a PCI-E graphics card.
If you really have no graphics applications, even the motherboard graphics may be good enough, and they’re cheaper.
The on-board (meaning on the motherboard) audio is usually good enough for most people, but you can get a really good audio card for not too much money.
Make sure the hard drive is big enough for your storage requirements. You might want to have two or three hard drives depending on your needs, but most people have just one. SCSI are currently the fastest/most expensive, but SATA is capable of being faster. ATA/IDE drives are decent, and what most people have in their computer today.
The motherboard will have a memory speed compatibility such as 333/400 or 533 (this is the Mhz at which the memory operates). Make sure you buy the compatible memory! It won’t work if your motherboard is 333 and your buy pc100 ram.
I prefer NVIDIA and AMD parts, but ATI and Intel, and many other manufactureres make high quality parts.
Sorry, I don’t have a website off the top of my head for how to actually put these things together. Hopefully one of the ones above did something for you. If you have any questions, feel free to PM me.