When I recently updated a machine, I replaced the motherboard, CPU, memory, video card and power supply. Money was a concern but I wasn't really limited, so I went for mid-high items because I wanted it to be a decent machine for a few years and that spreads the cost out. The old motherboard was really old and had AGP graphics, so the video card update was required. I kept my old hard drives, case and optical drives. All of this ran me about $1100 from newegg.com

Hmm. I've seen good gaming computers go for anywhere between $400-$4000.

Since you already seem to have the chasis, that's good, but it limits where you can go with this.

I'd say about $400 if you spend wisely, but don't fall for the cheapest items.

Try going to http://www.cyberpowerpc.com/ to see if you can build up something you like, and then figure out what it would cost you to mimic it.