Most Flash content / games perform kind of rough when playing them embedded on the page, but if you save the swf, launch from the file manager and adjust the quality to low, a lot of them play smooth with sound- even the 3D ones (which proves Flash uses the 3D on the N900, BTW).

A lot of games require some form of mouse action (click action or for directional movement). Problem is that when you play the files through file manager, the stylus simply moves the whole screen, rather than interact with the games.