If carbed to style you're aiming between 2.0 and 2.6 vols.
As for pressure, that's dependent on temperature. Refer to this chart to figure out the pressure to use...
For technicality sake, you should use nitrogen/mix. For guinness using co2 youd prob want something pretty low. Like around 2 or so volumes, not higher imo (probably lower). Thats considered low/under carbed for most beers. I cant really add anything else to this thread right now. sorry...