If you're wanting to make good pictures without having to mess with your images on your computer a lot, the D5000 would be a better choice IMO. The D90 is larger and slightly heavier which makes it less handy for baby pictures where you never know when you'll need the camera.
As to the Ritz vs. Costco question, it would depend on the staff at the Ritz store. If they are knowledgeable and helpful, I'd go with them. If there isn't any 'value added' by the store, then Costco is great due to their guarantee.
B&H is good, and you might want to check out Berger Brothers, as they are a site supporter.