If you’re not a tech geek or computer hardware wizard, you’re not going to be able to piece together a Bitcoin miner computer system that is cost effective for mining Bitcoins. Like just about ANY tool, one that is designed to do a specific task is going to be far more effective at that task than one that is designed to perform a wide variety of tasks.
Butterfly labs has designed a bitcoin miner specifically for mining Bitcoins. Their hardware is currently available for pre-order in 4 different sizes: 5 GH/s, 25 GH/s, 50 GH/s, and 1,500 GH/s. these systems are the best miners on the market, and have just started shipping this month (April of 2013).
There are some of these Bitcoin miners available on ebay as well.