I currently have a pair of GTX 570s running in SLI. Mobo has a 3rd pci-e slot that is currently being unused.
I have an 8800GT just sitting around and it's a nice single-slot card that won't take up much space. Power supply (1000w) is not an issue either.
Will there be any benefit to adding the 8800GT and dedicating it to PhysX? As mentioned, I have the mobo space and power to do it, as well as a spare card lying around so there aren't any logistical issues to overcome or extra $$$ to be spent. In other words, sans the higher power draw, it appears to be a mostly "free" option.
The question is whether there will be any real benefit - either now or in the future.
First, do you play any of the games that use a dedicated card? There aren't many, so if you don't play any of them it won't help you at all.
Two GTX570s should be strong enough to handle games and PhysX. I suggest trying it and reporting back. My gut says it won't help, but I could be wrong.
That's not how it works. PhysX requests go through nVidia's software. What's on the other end doesn't matter to the game. That's the whole point of the driver. So you can choose to have a dedicated card or not regardless. The real key is choosing a dedicated card that doesn't actually slow you down. The rendering has to wait on Physics calculations. Here's a video of what I am talking about, but it doesn't actually get to helping you choose which card.
That's not how it works. Yes there is a physX driver that is more widely used for many games, but those calculations are only done on the CPU. GPU accelerated physX has only been implemented on a very small number of titles, it's a feature that really hasn't taken off; nvidia can't afford to bribe every developer to use GPU accelerated physX after all. The only major games I can think of that actually support GPU accelerated physX are: Batman Arkham Asylum, Batman: Arkham City, Alice: Madness Returns, Dark Void, Metro 2033, Mafia II, and Unreal Tournament 2007. In total there are only about 25 games that can use GPU accelerated physX.