Network Diagnostic Util?

Currently we are running a small network with 3 locations (the main one being the hub) and approx. 30 – 35 users. Our connection at the main office is a 3mb up/640k down dsl connection. Since switching to a new enterprise package, we have been experiencing issues with latency and speed issues in general.

Our main office has been suggested as the bottleneck and I’m looking to get some quantitative data to help support this theories validity (or otherwise). My question is this:

Are there any recommendations as to which utility I should use to check the bandwidth at our location?

Regards,
Terry v.