upgrade "build" vs. 2.1 OS?

[jfolliard]

Forgive but I'm new to all this. I've been reading posts about the 2.1 upgrade to the operating system which recently came out - not sure if I want to do that just now.

But then I stumbled across a "Rom Update" on the HTC site which came out last fall. it's 2.73.405.5

My "build number" is "1.56.651.2 CL85027 release-keys"

So what's the difference between this Rom upgrade and the 2.1 OS everyone is talking about. I bought my Hero 30 days ago so it seems strange that I don't have that newer "build" whatever it is.

Should I upgrade to this new ROM?

Advise please?

Jack