The JSS contains some hardware and software change history for every managed Mac. Installed RAM and any changes to that are recorded in it, but I don't know how to tap into that for a report. You could view it manually for every system though.
If I'm not mistaken I think there was also a custom report file you could grab for JSS version 8.x that might give you a hardware change report. I seem to recall something like that back on the old Resource Kit, but I may be imagining that. Since those types of custom reports don't exist in version 9.x anymore, it would only be useful if you were still on some version of 8.
Outside of this, while it definitely wouldn't help you out post upgrade, you could also do this - make Smart Groups for your Macs that contain certain base amounts of RAM, for example, one for 4 GBs and another for 8 GBs. Then set each one up to alert you via email on changes. If someone adds more RAM it will report it to you since it should fall into, say, the 8 GB Smart Group, or better yet if someone steals some RAM form their Mac, it would fall out of whatever group it was previously in. Again, not helpful after the fact, but it could help catch those cases pretty quickly after the change was made (as long as you actually read those emails :-)
