More cores doesn't mean quicker in every area
An unclocked Intel Core i5 2500K - 3.3GHz - 1MB L2 - 6MB L3 will outperform a AMD Phenom II X6 1055T - 2.8GHz - 3MB L2 - 6MB L3 in almost every area, particularly 3d gaming if that's your thing...
Anyway none of those Dell systems will be pants it's just you are paying quite a bit for some pretty ordinary parts i.e. the graphics cards