Is this the entire system-tower, monitor, mouse etc, or just a new tower?
Avoid prebuilt systems for gaming almost all of them use low output power supplies, which means they cannot handle a powerful video card and only a few of the more expensive ones include a decent gaming video card, so you'll need to upgrade it pretty well out of the box, adding to the cost and voiding the warranty.
Try looking around the local, small, computer stores, they'll usually offer a custom build service so you can get exactly what you need.
If you lack knowledge on how to spec a PC, post in the Homebuilt or Components Forums for further advice.