Don't go for an AMD CPU if you have money to spend. The chips are physically larger, develop more heat, consume more power and have drastically lower performance pr. core; which is still what matters in almost all games. AMD CPU technology is a lot less sophisticated than the i-series, and the i-series is only priced marginally higher than similar-ish AMD CPU's. Moreover you won't get PCI-E 3.0 with an AMD processor. I'll wager my money that you'll get better perfomance in BF3 on a high-end i5 or i7 and a matching PCI-E 3.0 card than you will on any similarly priced AMD CPU, even though that particular game is one of the few that can take advantage of 6 or 8 "true" cores of an AMD CPU.
Check that Guide from Tom's Hardware, they know their stuff.
And you probably will be playing BF4 when the hype has peaked and the BF3 servers start shutting down, just sayin' It's impossible to like EA, but the BF series is the best shooter series there is.
Also if you really got money to spend, you could consider getting a 23-inch screen, they are cheap. I got an ASUS VE247h which isn't spectacular in any way in terms of features but is cheap, have a nice image, 2 ms response time and full HD. I think it's about 120-140 quid in the UK.
I would have found some prices for you, but I don't know where to look in the UK.
EDIT: If it was me, I would do like this:
Mobo: Asus - Sabertooth Z77
CPU: i5-3750K + Quality aftermarket cooler like Cooler Master Hyper 212 Plus
RAM: 8gb Kingston HyperX 1333 MHz DDR-3 RAM
HDD: Samsung 840 Series 120 GB SSD
and:
Western Digital Blue 1 or 2 TB SATA 6GB/s
PSU: Seasonic or similar quality producer ~750W unit - "80 Plus Bronze" quality rating at the least
And then get a 23" screen and/or save the money for a high-performance PCI-E 3.0 gfx card 6-12 months down the road, when the prices drop on things like GTX 690 / GTX Titan / HD 7970 or something entirely new is available. In a couple of years, when the system will struggle a bit with the newest games on high settings, I'd overclock the i5-3570K - it's dead easy, pretty safe and that particular processor is an absolute beast for overclocking.