Out of all those brands. Get the Dell XPS if you really want a gaming computer. Sony VAIO's are nice, but expensive. Dell's have gotten alot better, but customer service still remains the same (unfortunately). HP (which bought out Compaq) is pretty nice, they offer nice solutions at a good price.
For graphics cards, if you have the money, get the fastest of em' all. The nVIDIA GeForce 7800 GTX. If not, anything under that is pretty good for the price, especially the nVIDIA GeForce 6800 GT and the ATI Radeon X800 XL.
dell xps is a nice computer but i really like my dell 9100 because i tricked it out.....from the dell site of course, and i have the ati raedon x300 graphics card and it serves me just fine