I don't know what the requirements will be, but if you're not in the beta yet I'd recommend signing up for it. You can download the beta for Server 8 and test it versus 7 on your current setup. That should allow you to do some benchmarking.
I do recall that upgraded viz engine can make use of OpenGL, but that may only be relevant for the client hardware. Still it might be worth checking for OpenGL support on the new system. Other than that, 8 is supposed to be more efficient than 7 so I wouldn't think the requirements would be much higher, if at all.