I'd wait. Following the formula (I could be wrong), but every x80 part becomes the equivalent of next gens x70 part. While I know it won't be the same chip in the 870, it will probably perform around the same as the 780, and the 880 will no doubt out perform it, and at least be on par with the 780ti. Let's just hope NVidia don't decide to be their usual selves and release the 880 at a £600 price point.
EDIT: Just bought a Seasonic Platinum 660W as a gift to myself, hurrah. Now I pray that my car doesn't cost too much to repair...