The decision is yours. Do you really need a card now? Buy the 7950 that can be found for as low as £210, it's a bargain at that price. Do you prefer to wait 2-3 months and buy an overpriced HD8000 series during the first months? Or do you prefer to buy an HD8000 series few months after it is out and thus at normal price and with better drivers?
That said, Carspace above wrote the right thing to do. You buy the best available at the best price/performance at the time of your interest. If however you are looking purely into performance (without caring for money)... you could just wait a couple of months for the new cards.
Thanks everyone for the responses. I'll probably buy the 7970 and if the 8000 have been released, hopefully it'll just be cheaper.