BF3 and Arma 3?

£700 will get a very decent mid range system. something like a 3820k 4-8 gigs of ram and a gtx 560ti or better. as long as you dont go mad on motherboard selection and buy something with 4 pci-ex16 slots. you will have to scrimp a little on the case but coolermaster offer some very decent cases for about £55 spend about the same on a 550w psu (some cases come with a coolermaster cx 550w for about £70 but they are often snapped up so it will be luck if you can find 1).
but yeah spend as much as you can on the basics of psu, motherboard, cpu, ram and gpu and fit the rest to your budget.
i often find its easier to set funds for the parts and see what you can get for em.
case 60
hdd 70
dvd 20
cpu 170
gpu 150
motherboard 80-100
psu 55-70
then if you have a little left over consider cooling and maybe a slight splurge on a bigger gpu.