I have a D5000 (my first dslr) and I am very happy with the IQ it provides. I have learned a lot with this camera. Like you, I'm considering going to the D7000 or D300s for two reasons.
1. I find the view finder is too small and makes framing a picture somewhat difficult. Does your wife find this to be a problem? Also the more I use the camera, the more I want external controls and a larger body.
2. My wife is starting to go with me on photo adventures (mostly wildlife) and she would like her own camera. She's not bothered by the view finder so the D5000 will become hers.