I'd guess they're ALL "capable." Are you talking about stuff like the ChiliSoft add-in for Apache vs. IIS, or are you asking "how many IIS users have a database installed that their web server can connect to?"
The biggest problem you're going to have is WHICH database they have. I your application is simple enough you MIGHT come up with generic enough ADO and SQL syntax to make it somewhat database-portable, assuming you rely on System DSNs or the user configuring connection strings (bleh!).
If this is to be an MS-only solution you could package MSDE as part of your install package - assuming you have VS 6 (and thus redistribution rights). Of course MSDE may not handle the volume users might have, but they could then substitute SQL Server with little or no change to script code.