This is equal to PCI-E 1.0/1.1 at X8.
That would mean any of the cards so far listed should run with no loss of performance.
There's an article on Toms about PCI-E bandwidth use but it's for the older PCI-E 1.0/1.1 spec, I'll try to find and link to it.
[This is an old thread, but Google likes it.] Those who have x4 or x8 PCIe slots but no x16 slots for video cards may wish to consider the AMD ATI Firepro 2700 x1 http://bit.ly/iJxbGZ. Worked fine in my aging Dell 2900 with dual monitors. Performance decent.