#1 I believe that out of the box is supports SDHC... don't remember for sure I had to get in to either my N800 or my Treo to get support
#2 & 3 check your application manager, under your settings menu I believe, also you can go to on your N800 got to gronmayer.com/it/ and check the install all that should open up your application manager where you install applications, or like Benz145 said you can check out maemo.org
#4 there a many players out there it all depends on what type of video you are trying to watch, I, like many others happen to use mplayer for video and canola for audio
#5 I am not sure about this one, I have upgraded to OS2008 and as far as I can tell it works on OS2008 but don't remember if I had to do anything to get it to work on OS2007