Theres no way you made a computer that plays Guild Wars on high for $300. There no way in Hell.
As for being on topic -
1.) Build your own computer.
2.) Alienware is more expensive than Dell, don't buy either.
3.) You MIGHT be able to get a gaming PC for $800. Things that could keep you from it; If you need a new LCD monitor, or if you want to run the newest games on mid-high settings.
For $800 without a monitor, mouse, keyboard, joystick, sound card (a sound card comes integrated into the mobo) and without an additional power supply, you can make a computer that will run the latest games at low-mid setting with decent framerates.
Thats the flat-out truth.