Hey dan.
Here is what you need to build the system:
1. Computer Case (Make sure it has plenty of room for air-flow.)
2. PSU (Power Supply Unit. Only if the case doesn't already have one, which I highly suggest you buy the PSU separate!)
3. Motherboard (Make sure it fits the case, your two main motherboard types are ATX and Micro-ATX. The Case technical specifications will list what type of motherboard(s) fit.)
4. Processor (Make sure it fits the motherboard's CPU socket-type. The motherboard will list what type of socket it has for a Processor, just make sure the processor you pick uses the same socket-type. Example: An Intel E8400 uses the LGA-775 socket type, the ASUS Striker II has an LGA-775 CPU Socket.)
5. Memory, otherwise known as RAM (This is the DDR2 mentioned above.)
6. Video Card (Make sure your motherboard has a slot for the card.)
7. Sound Card (While this is optional, it takes a marginal load off your processor allowing it to do other things keeping your system running smoothly.)
8. Hard Drive (If your motherboard choice supports SATA-otherwise known as Serial ATA-you should get a SATA hard-drive.)
9. CD-ROM/DVD-ROM reader/burner (This is required to install your Operating System (Windows) on to your computer. Once again, if your motherboard supports SATA, get a SATA version.)
10. Operating System Installer Disk (This is required to install your Operating System of choice. For instance, Windows.)
If you don't want to put the computer together yourself, I'm sure one of us will do it for you. (For an extra charge, of course.) This will also eliminate the need to buy your own Operating System Installer Disk reducing the overall price of your build.