What's your current build? You should just be able to upgrade the power supply and graphics card for $400 and be able to play any game decently.
yo first off, is part of that four hundo going towards a monitor & other peripherals?
Its not really possible to build a gaming PC less than $800-$1000 that can run modern games at higher settings with a decent framerate.
I call bullstuffEDIT: ok ill raise my budget to 600