Windows 7: Actual Release versus RTM version

Hi Folks
Can anyone tell me authoritively whether or not there any SIGNIFICANT differences between the RTM verson and the final 'across the counter' release version?
I have a copy of the RTM version (6.1 Build 7600) that I obtained some months ago, which is actually working perfectly well, although I get some 'nag' screens occasionally that don't really bother me.
Is there a meaningful benefit for me paying out $150 or so to get an 'official' copy, is my first question?
Secondly; can I do an UPDATE of my RTM version to the final (legal) version or do I have to do a clean install?
This is the thing that I want to avoid; you all know the agony of reinstalling all your applications AGAIN that is involved!
Thanks in anticipation
Merlin (South Australia)

The biggest significant change between RTM and 'over the counter' is the DVD case.

RTM is the Retail version.

The RTM and the retail release are one and the same. No differences, zero, nada, etc. The nag screens are probably asking you to activate your version of windows with a legal purchased key. If you fail to do so, your RTM (same as retail) installation will stop working or go into a reduced functionality mode, eventually. So the answer is you don't have to reinstall anything but you have to get a legal key if you want to legally activate your copy of windows 7.

Do they actually put it in a DVD case, or paper/plastic sleeve?

You gotto pay more for the case.