What is your familiarity with the nxx0 lines first off...
which ones if any have you used....
if you have never used the n810 before...you will find it delightful...especially if you can pick a few up cheap enough...it makes the worry about bricking one .....less of a concern...
I've been using my n800's and n810's since they came out ...back in the day...never ever bricked one or screwed it up to the point of it being not salvageable ...
Best suggestion is...do it right and you may not even have to reflash along the way....but it is good practice to flash and reflash...you can get your os back up and running lickety split....you will eventually want to mess around with it and put cool stuff on it.
Beware of the few neato things that states it could brick your device if you do not follow directions explicitly . Other than that...it's all way cool stuff.
Easiest method I do is here.....you'll get a ton more out of your device...when you feel more adventurous ...try penguinbait's console tools v1 I refer to....and put everything cloned on an sd card....it is more specific for the n810 though...my directions there are n800 specific for a few things....penguinbait did a version for the n810 clone to sd ...console tools...other people have posted methods as well...keep in mind ....read all posts on whatever specific method to get a feel for the benefit problems or downright difficulties with each method. Beware of the hazards of cloning to the internal 2gb on your n810...I advise against it.

I think this post here
http://talk.maemo.org/showthread.php?t=92502
will help you understand most of the process
you can use it all I think..just read what penguinbait has to say in the link I posted here to my instructions which then inevitably point to penguinbait's console tools v1...if you go to clone make sure you do everything for the n810 specific in mind and pay attention to those specific instructions.
I did another few posts elsewhere to try to bring together the best easiest methods for a new person to quickly get up and running in a very simple exact step by step .format
..It's floating around in the n810 n800 or os2008 subforums along with a few other how to's I did.

Read read read...
as much as you can..
then if you think you got it....
read some more...
then if you think you don't get it ......
shout out...
I and/ or others are happy to help.