Okay Swine after the online shop links you sent me iv'e come to this:
Between grafic cards i'd recomend either the 550GTX TI wich costs 120Pounds here
http://www.pcworld.co.uk/gbuk/evga-geforce-gtx-550-ti-pci-e-2-0-graphics-card-1gb-11336640-pdt.html
Besides the 550GTX TI i'd rather recomend the 560GTX but this will depend on your price buget
The 560GTX asus wich i have as well is 148pounds here:
http://www.pcworld.co.uk/gbuk/asus-nvidia-engtx560-pci-e-2-0-graphics-card-1gb-11284274-pdt.html
Since your upgrading your PC your gonna need that power supply....so i found 2 of them wich are both very interesting since their both corsairs(note: corsair is nowadays one of the top brands in PC components in RAM and Power supplies)
The first one is a 600W corsair with 2 6-pins wich are needed for your grafic card the 600W is at 50Pounds
http://www.pcworld.co.uk/gbuk/corsair-gaming-series-gs600-600w-power-supply-unit-07656696-pdt.html
The second one is the same version but it's simply 700Watts wich would ensure your computer is preped for any future changes as you told me on steam the 560GTX with your sistem will require a MINIMUM of 550Watts.
However the second one is 10 pounds more expensive, but 100Watts more power for 10 Extra pounds seem like a good deal...
http://www.pcworld.co.uk/gbuk/corsair-gaming-series-gs700-700w-power-supply-unit-07656749-pdt.html
So from my recomendation your looking at either 120Pounds +50/60 pounds = 170/180pounds for a 550GTX + 600/700Watt supply
OR
148Pounds + 50/60 pounds(in this case i recomend the 60) wich would be 208pounds for a 560GTX+ 700Watt corsair supply wich would "technicly" leave your PC ready for BF3 at full power...
Possible problems:
Your Motherboard may not support PCI Express X16 2.0 and only support 1.0
Your Motherboard may support PCI Express X16 2.0 but the integrated software may not suport it
Your PC case may be too small for the upgrade
Your Motherboard may be a piece of junk that needs replacement
Your Ram may be needing an upgrade soon
A noisy PC
Temperature increasement
I say MEH too much words on this post...soon people will call me dread....anyhow
MT