One method I've always liked is third party resources. Where resoures come from controlling an independent resourse node. Thus forcing player to compete over these noded to expand.

One good example is that of "workers" being independent units, with each town having a number of workers. That gathering resources or build structures for whatever player controls that town. In this way players can't build more resource gathers instead they have conqurer new towns to increase their work force. Killing enemy workers rather then capturing a town means depriving that town of its resource gathering potential and making the town less valuable should you capture it later. Of course a player could also choose a scorched earth policy of killing off workers in their towns rather then allowing them to be captured and used by the enemy.

I think the resources are both. I mean that it should be relatively easy and painless to set harvesters to harvest some resource just like it is to build an some sort of an extractor building there. You can always go after the harvester or the building that harvests it does relay differ much. I think there is far more difference between limited and limitless recourses.