Mafia 2 is a special case because unless you have a *dedicated* physX card, the cloth physX runs on the CPU, which is very taxing when there's a lot of NPC's around. This configuration makes it so that there's a much more significant benefit to a dedicated physX card than there is w/any other game.
And as I've said many times, a dedicated card is also a good idea when you have an SLI set, because physX can only run on one card. And since your cards in SLI need to be running at the same speed, what happens is that the 'cost' of physX on one card ends up being duplicated on the other card(s).
IOW, if the physX requires 30% of the resources of one card, you end with only 70% of the rendering power on all of the cards in your SLI set. It's not like when you're in (for example, 2 card) SLI, you'd get a 15% physX hit on each card, leaving you with 85% of both cards for rendering. Like I say, you only get 70% of each of the two, so, effectively, the physX 'cost' is doubled in SLI.
Edited by brettjv - 12/19/10 at 9:47am