Oh, well in that case...
First you must pick out a motherboard that supports all of the hardware you want to buy. So you need to look at three key components. The CPU, RAM, and the Video card. There are of course other things too like SATA and RAID support, audio, etc etc. But the three key components are CPU, RAM, and video card.
So let's start with the CPU. If you want AMD, get an AM2+ motherboard and if you want Intel, get a Socket LGA775 motherboard. Now there are tons of s775 motherboards, so you have to find one that supports the Core 2 series.
Next, you'll need to get some RAM. Most modern builds use DDR2 RAM, so that's what you'll need to buy for RAM...DDR2. Also if you look at the motherboard's memory standard, that should tell you what speed you'll likely want the RAM. In most cases, DDR2-800 RAM will work. Also you'll want to check the motherboard manufacturer website to see the brands that are supported. Some motherboards have particular issues with some brands of RAM.
Lastly, the video card. Any modern rig will use PCI-express for the video card. Some motherboards are PCI-e 1/1.1 where as the video card is PCI-e 2.0, or some motherboards are PCI-e 2.0 and the video card is not...but it makes no difference, it is all backwards compatible with eachother.
I hope that helped...