Quite a few people have been asking about self building a PC. Its a good way to get more bang for your buck. There are a number of websites and articles. I liked this article, it covers all the main points well. The only piece of advice I would give you, if you plan to build a machine is to set a budget and stick to it. Five or ten pounds here and there adds up to hundreds of pounds of over spend.
Also before you turn it on, check everything again (esp the graphics & CPU coolers) and look out for loose screws that might have dropped into the case. Lesson 3 in the article has some tests you can run if you can’t get life into your system.