So I found out recently that any PCI-e x16 video card will fit and function in a PCI-e x1 slot (just as long as it's not superbly heavy); it will just not run at full potential. That said, I'm wondering if it would be worth it to use a low-end video card, like one of these just for dedicated PhysX:
it would work but not at full potenial. An x4 slot would be better if you have one. and you'd pick up another ~10% performance increase. X8 over x4 made no difference and x16 over x4 was only another few frames per second, around 5% performance gain.
As for 'fit' - usually you have to cut the end of PCIe connector on the motherboard off as the x16 cards aren't notched to fit into an x1 slot. You'll see what I mean when you actually try to install the card.
edit - By the Way, a 512M card is plenty for PhysX. AMD says 256 and iirc a review said he never saw usage above 300mb. The 1Gb card would be easier to sell down the road thou.