So based on the excellent information above it looks like the U.S. total would be 132.8 million to date. This would take into account the more expensive doubles/triples and boxed sets by counting each unit. Which is how they are certified (e.g. PG = 8 million units / 16X Platinum). And that's all albums. I know singles count the same as an album in the sales totals but I think a 3 singles to 1 album is a much better comparison given the relative cost of each.
The U.S. is 30% of the world market now (I'm sure it was more in the past but that's the data I have). So if you tripled that total for the rest of the world you get 400 million. All agree, Zeppelin was most popular in North America so let's account for that by doubling instead of tripling. That's 265 million. Throw in worldwide sales of singles and the DVDs and 280 - 300 million seems a pretty good ballpark.
The U.S. and Japan account for almost 1/2 of all sales of recorded music. So if you're not big in one or the other or both it's difficult to see how you'd get too far up the worldwide rankings.