Help upgrading ram in my Dell Inspiron 15r N5110?

Hello all,

I am thinking about upgrading the ram on my Inspiron 15r from 4gb to 8gb because of things such as virtualbox, gaming, and photoshop. I just have a few questions.

1.) I currently have one 4gb stick and one open slot. Can I buy just one more stick of ram or is it best to get two so they match?

2.) I have an Intel HD 3000 integrated gpu and I was wondering if more ram would help to increase its performance.

3.) Can you guys recommend me some ram for gaming, photoshop, ect?

-Thanks!!