4870 2gb is a worthless expense, you won't notice ANY difference over a 1gb one except the cost. If you can spend £150, wait 2 weeks and grab 5830 when they are available, the only problem might be the price gouging going on, a 5850 SHOULD be £200 which will mean a 5830 will likely be £150-160, in which case it will destroy, by a pretty large margin, a 4890/5770/4870 aswell as giving you the newer features.
If you want to save money a good deal on a 4890 at £120-125 would be a good price, or a sub £100 4870 on a deal somewhere hopefully, I'd not rush into it and check daily/weekly deals several places and see if you don't spot some uber deal on something for a couple weeks till the 5830 launches. If you see a deal to good to pass up on something available now, go for it, if not you see a 5830 on launch and maybe grab one if in your price range.