The two months (gross) salary rule is a very good rule of thumb. Women expect something nice, its a devotion to them for the rest of their life. Yes, they do show off their rings to other women, and people. Its WISE to get the something they are proud of and looks good on them.
1 Carat would be the minimum , seriously. 1.5 is great, another more is gawdy in my opinion. Find out what the girl likes, you know when you are in the mall, stop by the jewelry store and see what she likes, before making a decision. Some girls like fancy stones, some like different cuts, some like simple ones. The size is easy to determine.
I'm sure if you two are at the state of considering marriage, you've discussed or window-shopped rings before. If not, perhaps you are too young in your relationship to consider marriage.