A home-built system will not beat a brand-name complete system in price. The real advantage is that you can get higher-quality, higher-performance parts in the exact configuration you want by going custom. For example, you can get a motherboard that will accept AMD's new 64-bit CPU. Even if you are not after this kind of power, you can buy ASUS or Intel motherboards that are greatly superior to the low-end generic boards you will find in an OEM system from manufacturers like HP, Gateway, and Dell.

Also, you have the freedom to upgrade your system in the future easily. This can be a real gotcha if you buy a pre-built system from an OEM. Most OEM systems including Dell use custom configuration that in many cases cannot be upgraded with other vendors parts. For example, Dell does not use the ATX standard for their motherboard configuration and has custom power pinouts which mean that you must only use Dell power supplies with Dell motherboards. If you don't know this you can fry your motherboard. When you build from scratch using top-of-the-line components you avoid the risks of customized OEM equipment entirely and thus you can do hassle-free upgrades in the future.