Buying a motherboard
-The motherboard is the foundation of your PC. Without the motherboard all else is pretty useless. You need to research the motherboard as much as possible and find the best out there. Now, spending 500.00 isn't very practical for a motherboard but expect to pay around 100.00 at the least.
What To Look For:
-First off you need to know what kind of CPU you would like to run. The most common and modern are Intel 775 and AMD 754/939 and. Socket 775 will make use of the newest Intel processors, while this is a new socket it brings with with next generation hardware such as PCI express and DDR2 RAM. If you are looking to keep using your old DDR ram and AGP graphics card you will have to look for a variation of this socket. There are different chipsets that support different things, 925X supports DDR2 and PCI-e, 915 supports PCI-e and DDR. You can also find socket 775 in an 865 chipset (which was used on the socket 478 Pentium 4) that supports AGP and DDR. So its up to you what you want to upgrade/keep from your old system.
AMD socket 754/939 is on the brink of a change over to PCI-express, but it is sticking with DDR for now. You will still be able to get this socket in AGP and DDR of course, but PCI-e is the future and if you are upgrading you might as well go with the new tech. As with all motherboards there are different variations of the sockets, called chipsets, each chipset has its features, its bets to look at the specs and read through what they offer and see what best suits your needs. The two most popular chipsets are the VIA K8T800/890/900 and the nForce3/4.
-You will notice on some motherboards that they will include sound and even more integrated accessories. Onboard sound has come a long way from the old days and is actually very good now. It supports 6.1 and even higher speakers in dolby surround. If you have a good card like the Soundblaster Audigy you would use this, but if you don't, don't feel you have to buy a sound card to get good sound. Other features include onboard RAID.
Asus and Abit are my personal favorites, both make high quality boards with a lot of features and accessories. The only difference comes down to when overclocking your CPU but that is a while different matter. You wont notice any difference between these boards when running your PC normally. I recommend either.
Learn More About Case's And Power Supply's
Learn more about motherboards
Installing a motherboard
Troubleshooting a motherboard
How To Build A PC | Different Types of Keyboards | Help with Hard Drive Installation | Computer Networking Basics | Introduction of Motherboard Functions | Computer CPU Processor Speed | Upgrade from Windows 95 to Windows 98 | Computer Monitor Troubleshoot | Partition Hard Drive | Installing New CPU Processor | Types of Network Cables