Depends on how many objects/services you would like to monitor and at what interval. If you can afford a quad core with 4G RAM would be good.
any where Nagios can compile and run except windows*, I don't know how things work with windows OS.
A working Nagios installation.
Without plugins Nagios is of no use.
Since N2RRD Package is written on PERL. Most systems have PERL installed by default.
RRDs and RRD::Simple
Apache web server.
(Optional) helps reduce load on system.
(Optional) some versions don't work properly with N2RRD package (test cases failed on 64bit intel based RHEL 6.x system)
Required by N2RRD package.
Recommended: Ver. 1.4.x as it has rrdcached, which helps reduces I/O traffic
for archiving performance data.
for displaying graphs from performance data stored in Round Robin Archives.
(Optional) for Visualization
(Optional) for configuring Nagios
Other Graph Tool