The higher ranks are pretty rare. Upon further research, it appears that--just to be confusing--Grant's "General of the Army" title was considered a 4-star rank, whereas Eisenhower's was a 5-star rank. Only four other men have been named General of the Army (George Marshall, Douglas MacArthur, Henry Arnold, and Omar Bradley). The only person to ever hold the title of General of the Armies while he was alive was John Pershing.
General of the Armies = 6-star
General of the Army = 5-star
General = 4-star
Lieutenant General = 3-star
Major General = 2-star
Brigadier General = 1-star