In answer too your Question
In looking at RM's the following are some of the major items that impact value and collectibility (not necessarily in any specific order):
1) Condition of the gun - there is a big difference in value between 95% and 99%
2) Configuration of the gun - is it in its original configuration and do all of the SN's of the parts match. Is the configuration an unusual configuration (uncommon sights, finish, engraving, barrel length, stocks, etc...)
3) Has the gun been refinished or reworked - if there is a star or a diamond or any replacement or re-bluing marks placed on the gun, this will negatively impact value. If it has been re-barreled with a post-war barrel or if the the rear sight has been replaced with a post war sight (something that I have seen on more than a handful of RM's) - that will significantly decrease value.
4) Does it have any of the "goodies" - which could include: box, certificate, certificate tube, sight adjustment tool, swab, brush, warranty card, warranty notice, grip adapter, factory letter from Roy Jinks, brown paper, RM brochure, etc...
5) Provenance - is there any documentation that ties the gun to a famous or infamous person or event?
And lastly - and perhaps most importantly, do you think it is cool and have the $'s to buy it, without getting in trouble with your wife and without sacrificing any of the necessities required by your family?
From someone who has fallen pretty hard into the pre-war magnum arena, I hope this helps.
Just for kicks - a couple of my favorite magnums:
Enjoy,