I am having trouble finding the specs for your PC,so can you post what power supply you have.But I think it is pretty low so that will also need to be upgraded.As for playing the newer FPS's and taking into consideration your budget, I would look at something like this.
And for your power supply this http://www.scan.co.uk/products/500w-corsair-cmpsu-500cxuk-builder-series-80-eff-120mm-fan-single-rail-atx
Making a total of £144.77. Now you most certainly can get a cheaper PSU but that would be a mistake, as a PC builder for some time I can assure you a decent PSU is an absolute must.
My choices are just a suggestion as you might want to go for an AMD card, maybe an AMD HD 6850.