The first computer is the best in my opinion. It features the best combination of CPU, GPU, and total price. RAM plays a factor too, just that it is not clearly specified on the webpage. These factors are the most important when choosing a gaming computer.
First of all, it has the fastest CPU. Clock for clock, it is faster than the AMD Phenom II X4, and it is also clocked higher. It is also a true quad core, where as the Core i3 is just a dual core with hyper threading. It may seem, that single threaded applications will run faster on the Core i3, but that is not entirely true. Also, having a CPU clocked at 4 GHz will allow the video card to really stretch its legs. That brings us between the Core i3 and the Core i5. For £8 more, I'd gladly take the Core i5.
Secondly, the 5850 is already recognized as a powerful gaming video card. Sure, it performs worse than the 5870, but for $70-$100 less than the 5870, it offers much more bang for the buck. Besides, since these are all high-tier cards, they will crush any game you throw at them. Just because it has lower performance relative to cards of its class doesn't automatically mean it is not well equipped for gaming. It just means more bang for the buck and of course more performance per watt. As for the GTX470, it is not recommend due to high temperatures and high power consumption, unless there is a good cooling in your case. Looking at the case, it does not have much airflow, and looks pretty compact. There is a concern that the 470 will make your case have high temperatures across the board inside. So the 5850 is the best video card in terms of a balance between performance, temperature, and price for a high-end video card.
Thirdly, their prices are mostly in the same range of £800-£900. It would be better to spend the £878 because compared to the other prices, it is much more worth it even though it costs more. For £67 more, the hot and high power consumption GTX 470 can be avoided, and the lower clock for clock performance of Phenom II 955 at a lower clock speed is replaced with a higher clock for clock Intel Core i5 at a higher clock speed. As for comparing to the third PC, for £8 more you lose a bit in terms of video card performance but get a huge CPU upgrade from two to four physical cores.
Therefore, the first PC with the fastest CPU, the second best high-tier GPU in terms of performance, and the best offer for the cost is strongly recommended. The CPU performs best clock for clock, and is clocked higher. The video card is high-tier card that will be more than sufficient for your games. Its most attractive features are performance (3rd, but this is a high-tier card), power consumption (1st), and price (1st). Since all the price ranges are clearly in the same range, with the max difference a measly £67 (but you avoid the hot 470 and lower performing 955), it is the best equipped for the price.
Take care.
EDIT: I'm worried that having a GTX 470 in an Antec 300 Case will have some overheating problems. The Antec 300 is a smaller than average mid tower case and the airflow is not as good as today's cases. The GTX 470 is much hotter than a 5850 at idle and at load.
http://www.anandtech.com/bench/Product/164?vs=158