I want to go up a few guages from 9's strings so i can play DROP A song's ect ect. I have a few questions...

1. What guage would be best for going to DROP A and still being able to go back to standard tuning?
2. I heard that question 1 can affect the nut of the guitar, here is my guitar

Would the nut on that be ok? Nothing has changed since i bought it. And besides these guitar's are aimed at metal guitarist's pretty much.

That's pretty much all i needed.

Thanks.