You should be fine to run Visual Studio in a VM - VirtualBox will shortcut most of the calls directly to the CPU so that most things run at near-native speed. Obviously there is some overhead, especially with regard to devices, and the host will consume system resources such as memory. But if you're running on a modern machine, it should run tolerably well. In any case VBox is very quick and easy to setup so it shouldn't take too long to test (installing Windows will be the biggest time consumer).
In terms of distro, Ubuntu is a good place to start, especially if you're not run Linux before. I've also heard good things about SUSE linux, and Debian is another solid distro that suits people who wish to tinker a little more than the average Ubuntu user.
64 bit Linux support has come a long way, to the point where you probably won't notice any problems at all with it. You are more likely to encounter obscure bugs here and there but unless you're planning to develop on that platform you should be fine. And you can still watch Flash (yay! Youtube!) if you install a 32-bit browser or use nspluginwrapper to wrap the Flash plugin.