I use an Athlon x64 dual-core processor myself and run 3 instances of wow on the same comp, fps gets a lil choppy but otherwise runs great without fiddling with core affinities; if you do have issues, try setting /console maxfps 30 or 35 to the back alts and /console maxfps 60(or higher) on your main, works awesomely for me, alts run fine at 35fps and my main gets upwards of 64-80fps in most non-shattrath places