As far as compatibilities go with older apps? Windows Vista 64-bit never gave any problems, nor does Windows 7. It's the 16-bit limitation that persists in many operating systems running on x64/x86_64 platforms that is the hindrance. Unless someone finds a way to emulate the environment while running it under a mode that doesn't require a full reset of the CPU (64-bit Long mode > 16-bit is NOT Native!), you're stuck with VMs.