Hello.
Top of the food chain regarding a LAN monitoring/fault managment platform is HP's Openview suite of software. The basic module is called NNM or Network Node Manager. I think when you buy this product (it's not terribly cheap I'm afraid) you get 500 node license out of the box if I recall correctly.
Everyone but everyone builds their respective fault management software to interact fully with HP Openview, inc. Cisco, 3Com, Nortel and HP of course.
You can get evaluations for this software as well. I think the basic package of NNM is pretty easy to administer when it's up and running and can do manual or automated device discovery.
Most organisations who take their infrastructure seriously should strongly consider this product. However, it's not for everyone (i.e. the cost can get quite high when you start buying additional licenses or additional modules). There are a LOT of modules available with this product. You can pretty much do anything with Openview at a cost.
There are likely free network management solutions out there (I can't really comment on whether they will be good for you as I have limited experience of them). Some I've heard of people using include Servers Alive, Net-SNMP etc. The things with free management software is that they may ultimately lack some of the features that you may need, may not be regularly updated if there are any bugs and usually there is no support path.
I hope this helps with your selection process.