for the "perl(Net::SNMP) is needed by nagios-plugins-1.4-2.2.el4.rf.x86_64", it means you must have the perl SNMP library installed on your system...
you may find it here:
search.cpan.org/~dtown/Net-SNMP- ... et/SNMP.pm
for "fping", I don't know.
But anyway, if you don't need these plug ins, you'd better try to NOT install them (by fiddling with the options).
Or also, don't bother with the rpm package, and get the .tar.gz source and compile them ^^
I hope this helped you a bit.
Don't hesitate to post again if you still have difficulties.