Supermicro H8SGL with PCIe x16 video card?

It should work fine. It will just run at reduced speed, but judging from a recent Toms review, even a GTX 480 will only be slightly bottlenecked even running at x4 speed.

Thank you.

Should it work fine because experience says it should, or should it work fine because PCI Express standards mandate that it should?

Both. The actual throughput doesn't approach the x16 in most cases. I'm actually using a pci-e x8 to x16 adapter in a Supermicro board with 2x quad core xeons and a radeon 5850 and it works great.