CraptacularOne

SLI does not make "one card faster" what it does is increase the rendering power available to a game by splitting the workload between them. Most commonly used today is a technique called alternate frame rendering. What happens is that if you took the frames and numbered them 1, 2, 3, 4, 5, 6...ect. The top card would be rendering the odd numbered frames 1, 3, 5...ect while the bottom card would be doing the even numbered frames 2, 4, 6...ect.

Having a card dedicated to PhysX will help but only in PhysX supported games like Batman AC, Mafia II and Cryostasis. However most do not make any use of Nvidia's GPU PhysX so if you play games like Battlefield or Call of Duty you will get no performance benefit since the PhysX card is doing nothing in those games.

Regarding benchmarks, I'm sure you're referring to 3Dmark Vantage in it's hugely misunderstood PhysX test. All that the test in Vantage is measuring is the PhysX performance of your selected PhysX processor, nothing more. It is not an accurate measurement of how your system will perform in a game. The reason is simple, say for instance you have 2 GTX580s in SLI and a dedicated GTX460 for PhysX. Of course the GTX580 is going to be faster at processing PhysX than the GTX460 as it has 512 cores vs 336. So when that test is run in Vantage your PhysX benchmark is lower than if you had selected your GTX580 as the PhysX processor. However therein lies the problem that most people don't understand. Your system, when playing a GPU PhysX enabled game will not be faster if you had left one of your GTX580's to do PhysX. It will in fact be faster in that game if you left your GTX460 to do the PhysX. Because in a game, there is much more going on than just PhysX calculations. The GPUs have to render complex 3D images, with multiple light sources, post processing effects, multi pass texture mapping....ect. And if you lump in PhysX calculations it's yet another task your GPU has to complete. It's also important to note that your GPU cannot process both PhysX and 3D game data at the same time. It must completely stop one task to do the other. Now of course this happens very quickly but it's yet another burden placed on your GPU. Removing this from your main GPU or GPUs and offloading it to a separate GPU can speed up the whole process.

Now on to your last question. Do you "need" a dedicated PhysX card? No, you do not. Your GTX560's are perfectly capable of processing both like I mention above, but there is a performance penalty for it. If you're playing games that use PhysX and play them a lot, then that's where having a dedicated PhysX card will make sense.

uncle poop

ok so physx is not in all games... and really not in games i would play... so this may no even be an issue... i'll just keep to the gtx560's and not complicate things like adding a dedicated physx card...

thank you for the explanation and help everyone...