Monitoring Multiple Servers (100+)

Currently, I’m maintaining 100+ Windows Servers (combination of 2000, 2003, Exchange and SQL). Im just new to this kind of setup. Previous I have handle 25 servers max.

Guys, I need your suggestion or recommendation on the following:

1. how can I easily monitor everything on this 100+ servers? (Hardware failures, server up/down, service status, processor, memory, HDD etc.)

2. what software do I need, to monitor and manage this servers? others suggested nagious & opennms. I hope somebody could share their setup with the same case. (budget $1k).

Any answer, would be highly appreciated.

Thanks in advance. Regards