1st: 2x4GB would be "better", but usually not in a perceivable way. Having less RAM modules usually allows for more aggressive clock speed and timing while keeping the voltage low. However the difference would not warrant spending more $$$.
2nd: If you just plug the RAM in, it will run at 1333Mhz so no danger for either the RAM or the MB. However, it also mean you could OC your RAM up to 1600Mhz without issue (unless from the MB).