Does a pci express x16 video card work on a pci express x8 slot? And if does is the Inno 8400/8500 work?

Yes, a PCI-express x16 card will fit in a PCI-e x8 slot. PCI-express x8 slots are usually the same length as the PCI-e x16 slot so the card will physically fit. The only thing is the PCI-e x16 car will not run at its full speed. Other than that, the card will work 100% perfectly.

And yes again, a PCI-e x16 Inno 8400/8500 will work just fine. If you can, I suggest going for the 8500. It is the slightly faster of the two.

Good luck!