If nVidia's control panel allows you to set a graphics card to be "PhysX" only, then thats exactly what it will do and you certainly wont need a SLI ribbon cable as they are only needed and only work with the same card (or family of cards).
As for whether 1x PCI-E is enough, I don't know for sure, but I would be very surprised if PCI-E 1x does not have enough bandwidth. Whether you will notice any performance increase or not depends on the game title in question and whether you can crank up the PhysX setting in game to max from their default options, most games don't even have such an option so this will be mostly title specific.
As far as I know, PhysX works perfectly well on most systems with a decent CPU (ideally a quad core) as most games are not well threaded at all and as such leave an awful lot of CPU headroom spare for such tasks. I bet that your best bet at getting more information on such matters would be to find reviews about PhysX being run on the CPU only, on a dedicated graphics card, or on the same graphics card that's running the graphics.
In a nut shell your best bet is usually to upgrade the graphics card that you have, its nearly always a better all-round option than going SLI (excepting the high end) and in doing so you will get more performance capable of running PhysX at the same time, might be better to do that that spend cash on a 2nd card for PhysX alone that IMO is unlikely to add a lot of performance.
Main PC, P180, CM Silent Pro 500M, i5 3570k @ 4.2Ghz, 8-GB @ 2,000MHz, 256 GB Samsung 830, 500-GB 7K500, MSI 660Ti Twin Frozr, PC is super quiet :o
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