you can get better performance than that of a 580 for only £200-£240 quid a 580 is £300+ you just need 2 x 6870 http://www.anandtech.com/bench/Product/301?vs=305
The 6870 doesn't come with 1GB iirc, BF:3 seems to be a VRAM whore, so 2GB is kinda necessary.