SDK is meant to be used for software development like porting, programming, testing. The NIT is an embeded device, and relatively slow for tasks such as compiling. Hence, you'd want to run your software on a faster computer. The VMware image gives you a Ubuntu 7.10 Gutsy Linux/x86 installation with the latest Maemo SDK already installed. The latter allows you to get a Diablo/x86 or Diablo/ARM development envornment.

For VirtualBox, while there are hardware accelerator drivers such as vmware VirtualBox uses a different one. You can always use vesa. Edit /etc/X11/xorg.conf and then you can run the command specified on top of that config.

There are good manuals for the SDK.

http://maemovmware.garage.maemo.org/README.html
http://maemo.org/development/sdks/maemo-4-1-diablo-sdk/