I'd advice against buying the latest of the latest. It's often too expensive and isn't that much of an improvement over the previous one. Only exception is the graphics card. There's currently a war going on between Nvidia and ATI which gives you great performance for really low prices. Get the Geforce 8800 GT. You won't regret it.What you should never buy is a QuadCore, stick with DualCore. QuadCore isn't supported by most games yet and usually gets worse results. QuadCore's only good for 3D modelers and video editors.
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