Save your money on a GPU right now - a previous-generation GPU (such as a decent 8600-series card) will save you a ton of money, while delivering upper- (but not top-)tier performance on almost any game that's out. Considering that GPU generations switch over as often as they do, it's better to stick to slightly older cards and just upgrade when you need to - you'll sacrifice a bit of here-and-now performance (but not much), but you save money in the long run.
That's just my opinion, however, and I'm certainly not as qualified on this matter as many of the posters here, but the bottom line is that you're not gonna put an 8800-series GPU in a computer you put together for under 500 quid unless you seriously skimp on everything else. Like vantwej said, you can always upgrade a home-built computer later with better GPUs and more RAM (if it's supported), but if you're building on a bad foundation (read: mobo), you can only go so far.
Bottom line, though: 500 quid won't build a computer that'll play current games respectably in 3 years... really, no amount of money would be likely to put that computer together today, because today's technology will be chopped liver in 3 years. Focus on building something that'll fit what you need now, and that has the upgradeability to last another year or two with an occasional facelift.