Been running Win8 for a while on a 1.6ghz Atom. The weak link in many Atom boards (yours included) is the 945 graphics chipset. It sucks, end of story. That said, Win8 is reasonably quick and smooth and quite usable. Honestly though, I'm not sure you'll see any real advantage from making the switch. Overall system speed (HD, RAM, Network) isn't really going to change with Win8, and boot times are determined nearly as much by HD speed as by the OS. It will probably boot a bit faster than stock XP or Win7, but if you're using sleep modes, the difference may not be very noticeable. What I like about 8 is that it handles new hardware really well. Much better than Win7 did at this stage of release.
There are a couple of other threads arguing the pros and cons of the OS in general, but on your particular hardware, I'd call it a coin toss.