Are you dead set on buying a tailor-made machine? Considered building one yourself from components? Usually better to build if you have the confidence, usually works out slightly cheaper and better specced.
Avoid PC World, Dixons, Currys, Tiny, Staples, Office World et al.
Many online suppliers or good independent local dealers will supply you with a fine machine within your budget.
For 2.8Ghz you're probably going to have to choose Intel but CPU speed isn't everything, an AMD machine running at a lower speed is likely to outperform a 'faster' Pentium based setup.
What will you be using the computer for primarily? Graphics work, Games, Office work & net browsing mostly?
If you could outline your main requirements, it would help us make recommendations.