Originally Answered: What is a good gaming computer?
Most games are still optimized for dual cores, but having other processes running, and recording (recording uses alot of cpu).
Id recommend a 2.8ghz+ quad core processor.
You really only need 4GB of ram, even for gaming and multitasking, but if you want to future proof more, 8GB is good. More then 8GB is plain stupidity.
Now as for graphics, to play cod MW3 on highest settings @ 1920x1080 will require a higher end graphics card like a gtx560 or 6850 or better. So id suggest getting a 6850 or higher graphics card.
EDIT: Well it depends on what you need. If you need operating system, monitor, keyboard/mouse, speakers/headphones, $500 isnt going to get you very far.
For a decent gaming computer you need $500-600 for just the computer tower itself, none of the above things.
Its also usually better to build your own computer. Also if you do that, you can reuse an old HDD or dvd drive and save a few bucks aswell.
some cheap z68 motherboard
4gb of ddr3 ram
500GB-1Tb 7200rpm HDD
Any sata dvd burner, they are like $15-20
a cheaper $40-60 mid tower case (antec, coolermaster, nzxt, thermaltake, rosewill cases are good)
a 430W corsair PSU
an amd 6850 graphics card
That should run somewhere around $500-600.