Quick disclaimer, my wife never asks for anything, and deserves everything, and at the time we got engaged, I had a decent income and no real bills to speak of.
I went overboard on the ring, and obliterated the 2 months salary rule. Remember, the size of the rock and the price tag at the end do nothing to measure your commitment or level of love. Due to the luck of my situation I was able to buy the ring of her dreams, but it came at a cost of over $12K. It currently appraises for more than double, and is the nicest thing I have ever seen. It is too much for anyone, but the closest thing I could do to get her what I felt she deserved.
Some advise, take her shopping and tell her you are thinking about things, and take note of the settings that she spends the most time looking at, but do not be obvious. When you know the setting, pick a stone you are comfortable with to compliment it. Never pay the marked price, and if you can pay in cash... do it! Also, be sure you talk to her dad. It is old school, but gets you mad bonus points. I did it prior to actually buying the ring, but make sure you do it before asking her.
Also, my brother got the warantee or whatever in case he had to return it, that was a good move for him. I would say look into that if it is offered. You never know.