Physics cards

Perhaps 1x would be limiting, but I'd assume 4x and above (especially if PCI-E 2.0) would not cause issue. What sort of Physics cards are you referring to? PhysX cards, or something else? If the former, just consider it a low end GPU.

Well, here is a site showing a comparo of CPU PhysX, PhysX on the rendering card, and PhysX on a dedicated card from a 8400 to a 9800. From the looks of it, not much is needed. A 9600 looks like the approx 'sweet spot'. A super high end dedicated doesn't seem to add much, but I just glanced over the results, so you probably can gain more by looking more closely. However, if only a 9600 is needed, I think 4x (maybe) or 8x (or more) is probably all that is necessary.

Edit: Link: http://www.pcgameshardware.com/aid [...] ws/?page=2

Well, for 1.0 vs 2.0, you can just (approximately) double the x## to go from 1 to 2 (since 2 has 2x the bandwidth). As for the 1x, 2x, 4,etc., assuming it follows the graphics card trend (which since they are graphics cards, though they may be less bandwidth limited, so this link may be the worst case) slightly older/not top of the line cards did quite well on PCI-E bandwidths below x16 and even x8

Older, so 1.0:
http://www.tomshardware.com/review [...] ,1572.html

Newer, so 2.0:
http://www.tomshardware.com/review [...] ,1915.html