8800GS has 65mn GPU and 3850 has 55nm GPU. SO the 55nm can be oced much better.
And in stock speeds also, 3850 beats 8800GS hands down.
And with newer catalyst releases, the performance of HD 3xxx cards are increasing like hell. The performance is limited only coz of driver limitations and no game optimaisations done for ATI.
Theoritically HD 3850 has 320 shader units where even 8800GTX has only 128. But it not optimally used by games due to lack of optimisation, and nvidia seems paying every developers to optimise for nvidia. See the "way its meant to be played" logo on most of 2days game launches?
But ATI is trying to retify the lack of optimisations thru gud drivers, nd they are working gr8. eg:
catalyst 7.11 reviews are the one available whole over net. But with 7.12 things changed gr8ly. But with tha last week release of 8.1, the performance has improved very much.
And also with the HD 3850 cards u get an HDMI connector too and its a DX 10.1 card and has PCIE 2.0 interface which is backward compatiblw with 1.1.
nvidia's DX 10.1 card aka 9600GT is just on its way but as of reviews available, 8800GT is better.