Get a build-your-own special from Fry's.
They usually run a big ad in the Friday newspapers.
I built a system for a friend for about $320 that included a case & PSU, motherboard, Sempron CPU, RAM, 30GB HDD & DVD/CDRW drive from parts at Fry's.
You might need additional parts like keyboard, mouse, monitor, OS, etc.
I only suggest that because you live in LA and there are Fry's in your area.
Have you looked in the FAQ yet?
How about the Wiki?
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