I will not be able to confirm this for about 12 hours, when I am back in my computer shop, but I think this is the case:
A PCI 16x card will work in a x1 slot, albeit at a slower rate. This should not affect performance at all, except for 3D games.
Observe the picture below (GeForce 7800/512MB, Dual DVI, the latest & greatest)
The red-circled part of the interface I think is to fit a 1x slot - If anyone else knows better, please post & correct me. I will test this as soon as I get to work tomorrow afternoon, I will post results here.
dave.