Hi Mark, welcome!
If you are looking at a ring with three stones (main + 2 sides), a rule of thumb is to keep diameter/size (not weight) to about 1/2 to 2/3 of the main stone. This in turn means price (assuming all else: colour, clarity, cut is the same) is much less than 1/2! A good starting point for budgeting purposes is about 15-20% of centre for both sides.
Here is a recent thread with a bit of a discussion around this topic
http://www.diamond.info/forum/index.php?sh...ic=5146&hl=
At the end of the day, here as in anything else, what matters is what she likes. The same applies to princess cuts. I don't like them at all, so I wouldn't ruin a pair of nice rounds by making them sit next to a princess cut. Some people love them. Neither one of us is "right". Mixing cuts can be fun, and some of the greatest pieces of jewellery exploit the difference in looks of different cuts to great effect. Sticking to one shape and cutting style is safer, though, and at the end it goes back to personal taste. I have a friend that keeps saying she doesn't like mixing step cuts with brilliant cuts - but then oohs and aahs over the pieces I have, and those are mostly mixed.
A couple of comments in terms of size: bear in mind that weight is only a rough indication of apparent size. Rounds will look larger than princesses for the same weight, and a well cut diamond will look larger than a poorly cut one of the same weight. If you want something stunning, my advice is to select a shape, then focus on cut first, colour second (and there's no "better" here - only more or less expensive), clarity has to be eye clean (but don't "waste" money on anything above VS2), and adjust weight to suit budget.
Lastly, since you are at the beginning: by any means ask questions and form ideas and opinions, but the three most helpful things you can do are:
1. Go and see diamonds. Real ones. Well cut and poorly cut. D colour and K colour. VVS1 and SI2. Oval and Emerald cut. 0.50 ct and 4.50 ct. And everything else in between.
2. If you can, go with her. I'm still surprised at what my wife ends up liking or not liking, and I've lived with her for over 20 years.
3. Before you start choosing stones, choose your dealer. Talk to people, call them on the phone if they are remote or they deal online. Get a feeling for whether you could get along with them, whether they are interested only in your money or in helping you to make a good decision, whether they are truly the expert that they claim to be.