I have switched servers a couple times. Pretty much everything is in the Database, so you don't need to backup much of anything. (as long as it was done through Solarwinds approved ways)
I change the web.config:
<add key="TabsMouseOverTimeout" value="300"/>
<add key="TabsMouseOutTimeout" value="700"/>
<add key="TabsSensitivity" value="1"/>
That doesn't get stored in the database, because I use notepad and edit the file directy.
Oh, NCM and scheduled tasks... I don't think the scheduled tasks come over.
Also, I would backup your reports directory, I don't know if that comes over (if you don't have any custom reports, that doesn't matter.
Might as well keep a copy of the program directory and the www root folder, then you don't have to worry.
If you edit any htm directly (I did that for a while, but i don't need to anymore) you need to back that up.
I am running mine on server 2008R2 now (from 2003 x86), I would suggest you go as far up as you can so you don't have to do it again for an OS.