For gold, I did a web search at www.google.com, on "total world gold reserves" and found a webpage at World.Gold.Org that lists the world's reserves as of March 2002 as 32,692.3 tonnes. One tonne is a million grams. Now you just have to divide this by the world's population which is about 6 billion and you get 5.45 grams per person. (There are about 28 grams in an ounce.)
You could do the same exercise with other elements like silver and platinum.
A similar websearch for land areas found this page on World Geography which claims the total world land area is 148.94 million sq km of which 10% is arable land. Doing all the calculations I come up with about 0.0025 sq km of useful land per person, or 2,500 square meters per person, which is a plot about 50x50 meters. So if all things were equal we would each have about half a soccer pitch and about a sixth of an ounce of gold.
Enough space for a small farm and about US$225 in spending money.