VMware Fusion is in beta for Mac, and should ship before August: pre-order it now and save money. I've been using the beta quite heavily (for Linux rather than Windows) and found it very solid. I was never particularly impressed with Parallels, and it crapped out on me a couple of times.
I can't help with switching, because my background is Unix rather than Windows: I've barely used the latter.
As for programming, if you want to do "general-purpose tinkering", you've got a choice of scripting languages: the Mac comes with Perl, Python and Ruby. I've done a lot of programming in Perl, but I'm now warming to Ruby, which is a cleaner language. And once you've developed your Ruby skills, you can use them for website development using Ruby for Rails (install the Aptana IDE and its Rails plugin). If you want to get fancy and produce natural-looking GUI applications, install Xcode (comes with your Macbook, but isn't installed by default) and get "Cocoa Programming for Mac OS X" by Aaron Hillegass.
Since the Mac is Unix under the hood, there's a huge amount of open-source software for it. There are two popular package managers which will make it easy to install open-source stuff and keep it up to date: MacPorts and Fink. This way, you could probably even stick with FORTRAN or BASIC, if you're feeling masochistic.
Invest in an external disk and a copy of SuperDuper! for backups.
http://www.vmware.com/beta/fusion/http://www.aptana.com/http://www.macports.org/http://www.finkproject.org/http://www.amazon.com/Cocoa-Programming-Mac-OS-2nd/dp/0321213149/http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html