It seems like the most common collecting model is to collect all carts (discs, etc.) for X system. Less often, you hear about collecting for only a genre within a system or across systems. For example, I like to collect shooting games. Who else here collects primarily within one or more genres, rather than trying to assemble a complete collection for a system or systems?
Also, for those of you who collect in a genre, what resources do you use to determine what goes in your collection? While it's usually not difficult to figure out the domestic releases, it can be a little harder if you also collect imports. In my case, I find that the "xenocide files" at http://www.classicgaming.com/shmups/ and the Shooter Database at http://www.starbase299.com/ (now online again) are a good resources.
How 'bout alla youse?