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Given the price difference vs performance difference it certainly isn't worth it.
Techpowerup have just reviewed two 3850 512mb models and they both perform within a few frames of the 3870. (the results below are for the powercolor 512mb model which is clocked at 720/1800 but can be had for under 120 pounds - I know this as I have one )
Crysis 1280 x1024 2xAA 8xAF
Quake wars 1280 x1024 2xAA 8xAF
UT3 1280 x1024 0xAA 8xAF
The performance gap shown in the above (1280x1024 is roughly equivilant to 1440x900 albeit not widescreen) is minimal considering the extra outlay.
Please note I have taken these benchmarks at random but only focussed on a resolution close to the ops.