The best GOP setting would be "1," as that setting eliminates inter-frame compression and its artifacts. Some folks report good results with a GOP of 3 (inter-frame compression across two out of every three frames), and that setting allows a little more boosting of the bit rate.
In either case, the bit rate can probably be set to CBR 2.5 with a fast card and a fairly complex scene.
Perhaps someone else can chime-in on ideal slice settings, etc.