Stuff of legends is a good place to start:
http://www.solegends.com/citadel/index.htm
The original RTB01 plastic marines all have Mark V helmets with pointed fronts and studded shooulderpads.
The next plastics were Mark
VI and multi-part, but with less detail than later multi-part plastics, these all tended to have smooth shoulderpads with a raised trim.
Then there were the three-part marines with the seperate bolters and backpacks with little or no variation, these are probably the least detailled of all the marines out there.
GW released a new edition of plastic, multi-part marines around the turn of the century that had a far greater amount of detail and options as far as mixing types of armour components and weapons; these still make up the largest part of the marine range.
Some later three part plastics have been relaesed that have a similar level of detail to the new multi-part marines, but obviously not the same amount of customisation; these are mainly found in the Battle for Macragge and Assault on Black Reach boxsets.