Sorry about the delay in getting back to you.
Yeah, you'll need WSS 3.0 if you are going to develop locally on your PC; if not MOSS 2007 will suffice if you work directly on the server.
Not sure about needing .NET 3.0 to develop workflows in SPD, but if you do it in Visual Studio you definitely need .NET 3.0 or higher. I have everything installed on my PC here so all my bases are covered. I would go ahead and install .NET 3.0 - it won't hurt anything and save you trouble in the future if you find you do need it.
Hope this helps! Talk to you soon!
- Jesse