does your budget include a monitor, speakers, keyboard ect..., or just the tower.
if $400 includes a monitor, your best bet would probable be an off the shelf system, and to forget about anygames other than very simple ones
i would suggest saving some more money, around $600 for just the tower alone.
try reading up
i just finished this book, not great for building a gaming pc, but it is very basic, and includes alot of pics
there are also website that build sytems out of parts you choose.
if you do go w/ an "off the shelf system", i would stick to these sites.