The only difference in functionality is an additional class generator supported in VS2008 (and later) versions.
Windows added multi monitor support around 1998, and VS had full support by VB6 (before it was even called VS). I even remember fixing a multi-monitor Intellisense bug in that timeframe. [The original Intellisense implementation would move the popup window off the screen with a mouse-down so that you could see what was behind it (800x600 was still common in that timeframe, so a popup window could hide a ton of stuff). This was done by simply shifting the window position into negative territory, which was always off screen until multi-monitor. When a monitor was placed above the primary monitor, this click-and-hold gesture would simply move the window onto the bottom of this other monitor. I had a good laugh at the discovery of my really cool trick for retaining mouse capture on a hidden window, but the user was baffled.]
You can float any of the NORMA windows by right clicking on the window header (even if it is a tab) and choosing 'Floating' for the window style. This will let you move the window outside the VS IDE onto any monitor you like.
The DSL diagramming surface uses different rendering engines in the different systems, so you might see some slight differences. For example, the model note size is slightly larger in VS2008 than in VS2005.
As long as you get a non-express version (Standard or higher) you should have a very similar experience. Frankly, I still spend most of my time in VS2005 because I'm constantly relaunching it to debug the tool, and the launch time has gotten progressively worse with the latest versions. VS2010 leaks a ton of memory on my system, often getting up around 1GB of memory when I leave it open for more than a couple of hours.