Biketool is correct....TightVNC is a convenient way to achieve the desired effect....if that is indeed the desired effect...
I have used it a lot...

Frankly you aren't really asking "a" question ...pertaining to a specific usage...
But are asking many different ones and each has much diffferent effects in implementation and in end results...

Sure you probably could just use the n900 as a display...
Sure you probably could use the n900 and plug in usb sticks with alt os's...

Problem is...the n900 relies on the usb for power...
Plugging in other things into it..and running goodness knows what ...
Will drain your battery quickly.
Even with bluetooth or wireless mice and keyboards...the battery will drain quick.
Even if you have an octopus of an adapter to try to feed the n900 peripheral devices (mice, keyboards, usb sticks...) and feed it power...
It will still drain...quick (not as fast as without ...mind...but still pretty rapidly..)
To work around that...you would have to find portable os's that use a minimal power footprint ...and also use apps that are minimal as well...and even then...

Frankly ...if using a device for such endeavors ...
I would suggest a n800 or n810...with tightVNC...
Bigger screen....
Usb port AND separate power port.
It is much more viable a usage situation ...than the n900

But that is just my suggestion.