Go get the latest copy of PCA and have a look at the top 10 charts. Similarly buy a few other PC mags for the UK market and compare and contrast.
Without some suggestion as to what you want to use it for we cant really make good suggestions tailored to you. If you post what you want to use it for we can suggest systems tailored for what suits you. ie lower end graphics card if you are planning to do DVD editing. All important to know.