Personally, if you don't want to overclock, go with the X4 645. Because it has enough raw speed and 4 cores. So gaming and daily tasks, or even gaming with background tasks should all be a breeze. Now, if you are adventurous, then get the 555, because it is a black edition. It can be overclocked with one simple setting in the BIOS. Also it has 6mb l3 cache, which will help you a little in performance also. And, if you are lucky, it can be unlocked to a 3 or 4 core CPU with your board. But do not consider that in the price because it is only like a 50/50 chance you can unlock it to 4 cores. Now, if you've got a little extra money, I would recommend the 955 black edition. http://www.ebuyer.com/product/190673
It is 4 cores, 3.2 GHz and the multiplier is unlocked, so overclocking to 3.8GHz on the stock fan is easy if your case has good ventilation. But if budget is the utmost concern, go with the 645 or 555.
Well, it is 100MHz faster out of the box. The 640 and 645 are not unlocked, so if you want more speed, you need to raise the system bus and tweak voltages. Any time you get the Phenom, the CPU is fully unlocked. So if it doesn't feel fast enough to you, then overclocking is a simple multiplier change that won't hurt your board or other components like playing with bus speed. So you are paying about 10 pounds more for 100MHz. Sure, it's not a killer difference, but it is a little faster.
And if you are not doing a large load of tasks, the 555 is 100MHz faster when in single or dual threaded operations. But 4 cores is better for multitasking. For doing one thing at a time, the 555 will win out.
I think the CPU area should be divided into 3 sections. Intel, AMD, and I'm not sure section... when buying new systems, lol . When someone wants to buy AMD, everyone recommends the double or triple priced Intel solution. When someone wants Intel, someone will recommend the cheaper AMD solution. If someone builds an OSX machine, everyone says buy a Mac. If someone wants to buy a Mac, everyone says Hackintosh is cheaper.
This is a battle of funds, not a performance per watt benchmark. We are talking performance per Dollar or this case Pounds. I like AMD and Intel both. So if they want AMD, help them pick the best one that fits into their scheme of things. If they want Intel, help them choose which one. If they don't know, then you can recommend based on personal preference or benchmarks alone.
I think for his budget it is probably better to get the cheaper 4 core over the more expensive dual core phenom. The dual core is clocked a little higher and is unlocked, but it's not guaranteed to be able to unlock the other 2 cores. Best to get the cheap 4 core if budget is the biggest limiter.
He wants a discrete GPU. Perhaps the 2500 graphics is OK, but a discrete card is going to be better. Also budget is the limiting factor here. The CPU you keep saying is 100 pounds more than the one he is considering.