Thanks, Praz.
Is that documented somewhere? My understanding (which is admittedly not very complete) of x64 system address map initialization is that enabling "Above 4G Decoding" should allow the BIOS to grant PCIe requests for 64-bit resources without forcing them upon devices that do not request them. Thus, I'm surprised that enabling it black-screens my BIOS if my GPU doesn't request any 64-bit resources. (I don't know whether it requests them or not, since that doesn't seem to be officially documented anywhere.)
I'm also curious about the fact that the black-screen only occurs at the point when I attempt to re-enter the BIOS settings screen. After enabling "Above 4G Decoding", the system POST messages and logo all display perfectly, so apparently the video card has successfully initialized. It's only when I attempt to re-enter the BIOS settings screen that the board freezes. Wouldn't that suggest a possible BIOS bug?