Ummm... you might be able to get away with Starter for that. The one other feature that could sometimes be helpful is \"presentation mode\" (which seems to apply minor tweaks to prevent sleep/screen saver/system notifications), but that is only available in Professional anyway, so it's not in question here.
Another thing I should point out; I don't think you save money versus getting the correct edition right off the bat, but there is this:
http://windows.microsoft.com/en-us/wind ... me-upgrade
Also, if you want a detailed description of what features are in what edition, this page is the best I've found:
http://en.wikipedia.org/wiki/Windows_7_editions
Not sure whether Starter edition's lack of multiple-monitor support would be an issue with a projector. It might just be possible to switch them.
P.S. If it is within your means to run 2 GB of memory on Windows 7, it is probably worth it. 1 GB kind of gets by for relatively light use but the experience on 2 GB is better IMHO. Same would go for Vista.