I can't answer for the N900, but I will say that many simple Flash games work on the N810. The N900 will have a faster processor, so performance will be less of a limiting factor. However, some games have action associated with mouse hovering and this is a fail on a touchscreen device. Also, some games require fast keyboard usage that may be hard on a mobile device. I have no doubt that you will be able to download them (although you may need a hacker method to do so) and play them either in the browser or a third-party app.

For more info, see this:
http://talk.maemo.org/showthread.php?t=14317