You might read this before attempting to build a computer.http://www.pcmech.com/byopc/
Also make sure your graphics card is compatable with the new motherboard before buying. Probably the easist way to build a new computer is to start with a prebuilt barebones kit Here are a couple that are good starts.http://www.tigerdire.....u=C460-1022 T
The two shortcomings in this barebone kit is the Nforce 2 chipset, which is already slightly dated, and that it has very few USB ports. The sempron processor is like a intel celeron, but that can be changed to a Athlon XP for a price. It supports up to a athlon 3000 processor. It also is limited to 8X AGP, but until SLI graphics cards start coming down in price, i really don't think this is a bad thing.
If you like Intel processors this one is pretty good.http://www.tigerdire...3&Sku=TC3J-2032
You are going to spend some more money for the graphics card(s) but overall this is a good start. I do like the USB ports at the top of the case.
If you would rather buy the parts and assemble the puter heres some sites i use and have had no problems with.http://www.directron.com/index.htmlhttp://www.newegg.com/Index.asp?page=1http://www.acortech....3...er New Pull
If you really want to check for best prices and also see reviews of companys so you can seehow well they are doing i would use pricewatch.http://www.pricewatch.com/
I would not deal with any company that is not 80% or better there. Even some of the better rated companys can be a bit slow shipping items you have paid for. Newegg is the class champ on fast shipping and they are the best on returning damaged items.