FileMaker Pro is an excellent suggestion. We're using FM Pro 8.0 and FM 8.0 Server at work, and it is sweet. However, we had to get a FM software solutions guy to build what we needed. Thankfully, he was willing to do some work pro bono since we are a nonprofit organization.

Our FM 8.0 Server runs on an Apple XServe G5, but our clients are split about 50/50 between Mac and Windows. We've got four sites that have to log in to the system and utilize the server from remote locations. Works like a charm.

Once up and running, it's rock solid, and it's almost idiot proof from the end user point of view.