Network monitoring/inventory

Greetings,

I am looking for some suggestions on network monitoring software. Here’s our scenario:

Approximately 50 subnets. Virtually all subnets have about 25 total devices. Our Corporate office has about 250 devices.

I need to track serial numbers for network attached devices, for inventory purposes.

Need to be able to update database as many as 3-times per day.

If you have actual experience with such a product, I’d like to hear some suggestions.

Brian