I have a question why does the 8800gtx have 2 pcie slots for power and the HD 4850 only has one, Can anybody tell me why?
Dual slot vs single slot has nothing to do with the power requirements of the cards, just the cooler design.
The 8800GTX has a dual slot cooler while the 4850 only has a single slot cooler.
As you already have the two cards, I would defiantly use the 4850.
Even thought the raw performance is comparable, the 4850 will draw less power (105w TDP vs 145w for the 8800 GTX), is DX10.1 capable and will have much better driver support and optimisation now and in the future.
Unless you are planing on exclusively playing games that are optimised for nVidia's hardware, do not pay to much mind to the whole 'it will play optimised games better' argument.
It goes both ways with some games running much better on ATI cards while others run better on nVidia cards.