hey Dave,
I have used nagios in loads of environments, and in many startups and even well established corporate banking and asset management business lines with the infrastructure size matching yoru description.
I would recommend nagios for the following reasons.
1.) There are two versions, one is open source and the other is commercial ( just the same product, fully polished with advanced dashboard, graphs , trends , reporting and analysis added )
2.) you can go live with the open source and then try them for as many years as possible and then switch to support. even during open source you have huge support groups...
3.) the commercial one, unlimited node license costs around 2.5 grand or less... thats the best value for such stable and tested product.. 50 to 100 node license are less than 1.5 grand i guess.
4.) runs on all OS/ platforms etc, and runs on secure ssh, no unsecure connections, and needs an agent install on all nodes.. which is one time setup.
5.) not just monitor , but you can also start stop and control services. best part is becasue of open source, you can integrate your in house services or inhouse developed applications as neatly asany commercial application. you can define a new application by simple xml, and then add the stop, start , restart and monitor scripts... and you are on boarded
so i really recommend this seeing your size. the open source website is http://www.nagios.org/download and the commercial site is http://nagios.com/.
heres the product manual installation procedure : http://assets.nagios.com/downloads/nagiosxi/docs/XI_Manual_Installation_Instructions.pdf
and here's few quick start guides :
http://nagios.sourceforge.net/docs/3_0/quickstart.html
@PS : i have nothing to do with the company, and certainly i am not advertising it. but in my experience i have seen foglight and other products, and have come to appreciate the elegance and simplicity of this product, and also the fact that for every small change we dont have to depend on vendors who hide the code... and still get enterprise class support important for banks..
hope this helps.
-- Chandan