Hey Mark.
This is a common scenario. I am guessing you are using ODC connections to databases and the limitations are driving you crazy. I have been there. Unfortunately a lot of those are still present with ODC connections in 2007.
Using a SQL backend is the best way to go. If you want to do enterprise form solutions right, the way Microsoft and other large companies do electronic forms, you want to look at using web services. That is where the performance and flexibility are acheived. It also will make your developement and maintenance of your solutions much faster and easier.
I would start by looking at the Database Accellerator suite. This gives you everything you need out of the box for SQL and Sharepoint integration. It has worked great for me. Especially if you are committed to InfoPath and doing a lot of integrated solutions. You get one tool set that you can use over and over again and have everything consistent.
Database Accelerator works the same with 2003 and 2007 and can allow you to begin creating solutions without wasting your time or missing a lot of the features. IP2007, and especially SP2007, are definitly worth it and have good features but your transition is not as dependent on what you can do or waiting for them.
I would recommend you have Qdabra support give you a demo so you can see what is possible and how it might work for your needs.