Tool to scan servers in a WAN for Release/SP level on W2K3 Servers?

We’ve got a relatively large (700+) server infrastructure, and many of our servers are still at SP1.
Of course, we don’t have an accurate record of every server’s service pack and release level.

Do you guys know of, or suggest a specific tool to scan objects in the WAN, and return the host name along with its corresponding W2k3 version/sp level?