The first thing to determine is which breed of CPU you want to use, AMD or Intel. Personally, I'm an AMD guy because I think they give you the best bang for the buck for almost any application, but it's something you'll have to determine for yourself. Your choice of motherboards follows from your choice.
Next, read the reviews on [AnandTech.com]http://www.anandtech.com[/l] and other good hardware sites. Then cruise the forums, including the Motherboards and CPU forums to find out what really works when lots of people try the boards you think interest you. You'll also want to decide what RAM you want.
Finally, take the time to go through the weekend ads for Staples, Office Depot/Max, Fry's, Best Buy, etc., and cruise through the Hot Deals forum and other deals sites to find out where you can get the best items at big savings... and I do mean BIG! For example, for the last few months, every four or five weeks, Office Max was offering 256 MB of Kingston PC2100 RAM for $9.99 after rebate. If you're not looking to build the fastest machine in town, that's a bargain for decent, reliable, name brand RAM. The same is true for hard drives, CDRW's, and other stuff you may want to include if the price is right. A little research will let you afford a lot more.