I wouldn't be getting anything too expensive at the moment simply because the new cards aren't far away.
Ignoring the rumours run by as far as I can tell only 2-3 known to be crap websites, AMD cards have always been due for late this year, the current thinking is Dec 6th release.
TSMC/AMD/Nvidia have all said today that 28nm volume production is underway, parts have shipped to AMD/Nvidia from TSMC, as in finished working chips, and TSMC has the HP aswell as HPL process going at the moment.
The only real issue is, exactly which cards are in production, most rumours have high/mid end AMD but Nvidia only doing the low end for now with high end miles away. Either way its likely we'll have at the very least the midrange next gen parts available before the end of the year, and possibly the high end, meaning probably something in the £150-200 range with a crapload more performance.
Its a shame, would be great if the new cards were out before BF3 but thats life.
Maybe pickup a second hand 5850/5870 for now, pretty damn cheap, very good value, won't lose a lot of value after the new cards launch, some obviously but not as much as a 6950/570/580/6970 will lose. If you aren't happy enough with the performance sell a 5850/70 on in a couple months and pick up a new gen card, overall you'd end up with far better performance for very little extra cost.