Some people are having problems with nagios 2... it sometimes hangs and it's quite difficult to notice it.... (this problem can be solved with some scripting) and somebody is having problems even starting it.
If you want to be on the safe side start using nagios 1.2, get the sources and compile them by the books
Upgrading to 2.x later isn't too difficult, some file formats changed but it's mostly a couple of lines to be added or removed.
Personally i'm now been using 2.x for 4 months and i (finally) got it working right...
the only tips i would give from my side are take your time, read all the docs and use the sources and not an RPM package.
Plan AT LEAST a week for installing nagios the first time and don't be scared at the idea of scratching everything and redoing the install if you have some problems... you possibly simply missed a step in the instructions, but it's often easier to do it again (mainly if you are starting and testing only a couple of hosts) than trying to find the problem....