I have yet to run across a consistently reliable booking engine for Paris hotels. The best I've found is HRS; however, I can often get better prices at many hotels by calling them hotel directly. Also, beware of some hotel booking sites for Paris -- they add a service charge (often a heavy one) to the bill, and it's not at all obvious that they're doing it.

The Opéra area is a good place to stay, but it's not in the 1st. My favorite Opéra-area hotel is the Scribe (now a Sofitel, unless it changed again); it's got a much friendlier staff and is more reasonably priced than the Grand. If you want to jump down a notch or two, you might be able to snag a decent rate at either the Libertel Caumartin or the St-Petersbourg, both on the rue Caumartin, which is especially well-located if you're planning on shopping the big department stores.