Trust Dell not to know how the 920 works with ram setup the way it is (its from Dell need I say more) ….go with the 860 and thats my input done.
I personally will chose the Intel(R) Core(TM) i7 Processor 920. The 920 has triple channel memory at 1066Mhz that will be much better than 1333MHz at dual channel.
Also the 920 has better features.
You can compare them here:
I would go with the 860. The cpu is faster than the 920. The memory is dual channel as opposed to triple channel, but the ram will be faster than the stock ram with the 920. The 860 also has lower power consumption than the 920, therefore it should keep lower temperatures and in my experience lower temps will mean a longer life and less problems. Hope my opinion can help you out, whichever you decide you won't be going wrong.
There is no doubt on the performance of i7 processor but which one is better among the category depends on the purpose you want to use it for. Why don't you go and join Intel Live Chat Event on 26th March @ 9:30AM GMT and get experts opinion there. More info at www.intellivechat.com. You can joint them on skype @ IntelLiveChat