Actually I've now read online that you can use both at the same time if VMware is set not to use hardware virtualization mode.
Then there really is no point to trying it out. Hyper-V won't install if you don't have virtualization enabled in the BIOS (at least it didn't in Windows 2008 and 2008 R2).
Without HW virtualization, you'd need to have a real monster of a machine to get good performance out of a VM. If it came down to that, I'd just build a super cheap Windows XP machine with a somewhat modern graphics card and install old games on that.
I've also now read that Hyper-V doesn't support hardware 3D so it sounds like it is still basically just Microsoft's Virtual PC product integrated into Windows 8 which means it is useless for games.
Yeah, I don't think Microsoft is going for the legacy gaming market with Hyper-V. I think what they are going for is server consolidation (Windows Server), testing and backwards compat (Windows 8).