To try and answer some of the questions - I used to have an n800 ages ago...
1) The browser is great, must faster overall and handles all sites pretty well!
2) Flash is acceptable, still not brilliant but you can stream BBC iPlayer music content perfectly and watch youtube without too much trouble.
3) No need for Pidgin here, with the built in conversations support for multiple IM providers using an additional plugin! To put it mildly IM and VOIP support is brilliant... Best experience on a phone/tablet yet as they are added in with your contacts and people are expanding the services continually - facebook chat is coming for example.

I dont use remote desktop or transmission but I believe there are n900 versions for both.

So far I have been very impressed with the n900, great device - there are some things that are lacking but these are being addressed at least by the community (ie MMS support)