I would recommend at least 256MB of RAM -- 128 will do ya, but the more, the better. Go to 512MB or beyond if you can -- that'll make the experience a lot better.
6GB is slim -- not much room for music, photos and whatever else, but it's plenty to get a full install of OS X on. I wouldn't recommend letting that drive drop below 1GB of free space -- the HFS+ hard drive format that Mac OS X uses can have problems if you don't give it enough free hard drive space (good rule of thumb: 15% to 20% free space, always).