Hi,
I don't think that you will be able to get both your Graphics cards working at the same time.
I am not up-to-date with Nvidia but, with ATI cards the recommended set-up is to use two cards of the same spec. If you use two different cards (ie a 4850 and a 4550) it will down-clock the faster card to the slowest one.
It might be worth doing a check at Nvidia to see if you can actually run both these cards at the same time but my guess is that you can't and the reasons might well be that they each need different drivers as they use different chipsets.
ATB