do some window shopping with her... get a feel for what she likes. She should be expecting the proposal to come soon...
ask her friends... show them some samples
finally, buy it at a place that has a good return policy, like bluenile.com. After the engagement and emotions have settled, ask her if she really likes it. If not, just return it for a different one she does. Don't let your pride get in the way because she'll be wearing it the rest of your marriage/life.
I spent around $4k for mine and got exactly what I wanted and she loved it.