on a slightly more serious note, I don't really believe in spending an arm on a leg on an engagement ring.
When I proposed to my wife, I was earning less than minimum wage (which didn't actually exist at the time), and I spent very little on the ring. Didn't mean that I loved my wife any less, and I've more than made up for it over the last 12 years or so.
Buy her a ring that you can afford, and by all means sacrifice a guitar to do it. But if the marriage is worth anything, she won't be doing it for a ring.
Mark.