@Methanoid: And I thought I was picky lol. I actually think that £125 is fair for the 4850.
Not that I would complain if it was even cheaper, but what's the cheapest 3870 you can find? I've not looked too hard, but £90 seems about right. Assuming £90, that's 39% cheaper than the 3870.
Going by this review, the 4850 is between 32.5% to 57% faster depending on whether you play with AA enabled or not. If we go by the average, we get 45%, which makes the 4850 better value (more fps for the £). Granted, if you never/rarely enable AA, then the 3870 will be better value. Personally, I tend to play with AA, so I would put more weight to the AA enabled benchmarks. So for me the 3870 will have to drop to at least £80 before I consider it better value for my use. And that's ignoring the premium you normally have to pay per increase. I'd actually say that using the 3870 as benchmark, the 4850 is underpriced at £125. Or, using the 4850 as a benchmark, most 3870 that I've seen are overpriced. I can't comment on the 4650/3850 though.