Hi hongzu,
Here is a link to one supplier that you can look at.
http://www.novatech.co.uk/novatech/pc/gaming/
Bear in mind that the systems you are looking at do not have a GUI, (Windows xp/Vista/Windows 7), you have to pay extra for that. They do not have a monitor either, (and in some cases they don't have a keyboard and mouse).
http://www.computerplanet.co.uk/systems/gaming/index.html
http://www.pcspecialist.co.uk/
These two supply systems with Windows 7 Home 64 bit. That is only the box, no monitor,no keyboard, no mouse.
Have a look around. Type in Bing or Google or whatever, 'Gaming PC' and see what comes up. Make sure that you read them very carefully, especially if you go for a custom system, (the price may only include motherboard/processor/ram/power supply), you then have to see how much a hard drive/GUI/DVD/keyboard/mouse and monitor costs.
As I have said in other posts, a computer that can run SSEx may not need to be as powerful as you think. My advice is to wait and see.
Regards, Dave