It would be nice to have alittle more info from you before people start listing builds at that budget.
Do you have any parts already that could be used? Disk drive, case, keyboard, mouse, ext ext
Do you have a monitor already?
What res do you plan to game at? In other words what kind and size monitor do you have or plan to buy?
What kind of games do you play and plan on playing in the future?
Do you prefer nivida or ATI?
Do you prefer intel or AMD?
What OS are you going to use, XP, Vista? Does the OS need to be included in the budget?
These are just some details that will really help the people help you get a PC fitting your needs and budget.
This should be copy pasted to all topics like this one.