Building your own or getting someone to build it for your best way to go. If this is not a option look for something with graphics card and sound card, intergrated ones can not be upgraded so stay away from them. Also stay away from any thing you can buy at best buy. Dell's and Gateway's are your next lvl of computers. I hear falcons are great gaming systems http://www.falcon-nw.com/ . But a buddie has a alienware it runs great for him.
If you need to buy a prebuilt, I suggest Gateway or Emachines. I would do what some guys here suggested and build your own computer.
Same old... custom computer, you spend less money for a better machine... If you can not build your one, djust talk with a nice computer stor nearby and ask for help...
if u got enough money to throw around then get the area-51 cuz ive never seen neware else where u could buy a 4ghz cpu but if u dont then build build build
Ok alienware is awsume dont let anyone tell u diffrent yes they are overpriced but to someone who says that to u say F*** off buddy i got an alienware. But if its gonna be hard for the money buy off some idian joint for so cheap or buy off somewhere in the middle and get good deal. You could make ur own but thats easy for me and most other ppl to say because we no how but if u dont dont buy all the parts and exspect to put it together buy a premade