Regarding mac users:
Yes, UE does actually support Mac. However, due to Apples EULA's, one can only compile UE for Mac while running certified Apple hardware oneself. This means that we will need to compile the Mac release on a Mac. (As we do now with planeshift already.) A problem with that is that we will need to actually have a Mac using dev to compile it. For this demo, we did not. For the final UE port, we will certainly go out of our way to find one.
Regarding the system reqs:
So far I've seen the demo run at good speeds on a quad-core AMD running at 3.2 ghz and a 1gb mem ATI card, so I imagine it will still run (to some degree) on much slower/older systems, especially with the graphics turned down.
In this thread you should be able to see some results of what FPS people get using what hardware. Final reqs have not been determined, and won't be until the game is ready for release. We will try to support as many different configurations of course, and UE does seem to run faster than CS on average and high end system, but I have not seen enough data yet to tell you how it performs on low end systems.