Visual Studio .NET is a good choice, especially considering your background. It offers great tools as part of the studio itself, and add-ons that support nearly anything you can clearly describe. Like all good quality, powerful tools Visual Studio is expensive but it is neither out of reach nor outragously priced for what it delivers.
Open source tools also offer a good way to build applications that work via the web. The problem is that your experience is heavily rooted in Microsoft and Microsoft's quasi-consistent view of the intended user experience and the tools needed to provide that experience. Unless you're willing to make a huge leap of faith and also change many of your pre-conceived notions about how to develop/design/use software, I wouldn't recoomend Open Source for you.
There are other toolsets that can be used to build a good web application. I know that Rudy loves Cold Fusion, and it can do impressive things pretty easily especially if you use the rest of the Adobe suite.
The short answer boils down to there are many choices, you have to find one the works well for you and then proceed to use it. Make the best choice you can with the information you can get in a reasonable period of time (don't search for months or years), then move on and don't second guess yourself. If new tools become available don't be afraid to investigate them, but spend at least three times as much time working as you do researching other tools!
-PatP