That really depends on what you consider "satisfying enough". On the one hand, even the resolution on the current Rift is "satisfying enough", considering that it offers an experience that simply cannot be replicated by traditional means. On the other hand, you'd probably need a 4k display in the Rift to really match the kind of fidelity (in terms of detail per arcsecond) peole are used to from their monitors.Question for Rift owners: do you think ~1920x1080 resolution for both eyes would be satisfying enough in Rift, compared to using a regular 1920x1080 monitor?
But generally, I think a 1080p display will solve most of the really crucial resolution-related issues of the current Rift (screendoor effect and UI/HUD legibility).