So you've made the decision that you are going to collect a specific series. What are the next step(s) that you should take? If you are going to form a serious, high-end collection one of the first things that you need to do is to examine comparable collections. As an example, if you have decided to assemble a set of Liberty Head eagles, it would make sense to know the grades of other sets that have been put together.
When it comes to 19th century gold, some of the old stand-bys are the Norweb, Eliasberg and Bass collections. It is very instructive to compile lists of the coins in these three collections as they pertain to what you are planning to collect yourself. For example, if you have decided to assemble a set of high grade New Orleans half eagles, you can make a spreadhseet of the relavent coins in these three collections.
Luckily, this information is reasonably easy to access. (Thank you, Internet...) On the PCGS website, the Set Registry pages list the "probable" grades of the Eliasberg and Bass gold coins (not to mention another pretty decent set, that found in the Smithsonian). The grades of the coins in the Norweb collection can be found in the three sales of this collection that were conducted by Bowers and Merena back in the mid-1980's.
Knowing what quality coins were owned by a great collection is important informantion for a new collector. As an example, let's sat you are being offered an MS63 example of a specific Liberty Head eagle. If the best piece Bass owned was an MS61 and Eliasberg only had an AU55, then the chances are good that this is a significant coin.
There are exceptions to this rule, however. Let's say that the MS63 Liberty Head eagle mentioned above is from a small hoard that was discovered after Bass or Eliasberg stopped actively buying coins. In this situation, the significance of the Bass and Eliasberg holdings are not as great. An example of this would be an 1894-O eagle in MS63. This date was essentially unknown in Uncirculated when Eliasberg was buying and very few examples better than MS60 were available to Bass. Today, because of hoards found overseas, this issue is scarce but not impossible to find in MS63.
If the series that you are goping to collect has a number of active Set Registry collectors, it is important to study the top sets that are on both the PCGS and NGC websites. Let's say that you are focusing on St. Gaudens Double Eagles. Without being congnizant of the best active sets in the Registries, you won't have a good idea of what grades you'll need to make your set competitive.
But there is much more to assembling a high-quality set than checking out Set Registry information. I'd strongly suggest that before you buy any coins for your new set that you invest a few hundred dollars in books and auction catalogs. There is no more important guide for the new collector than important specialized auction catalogs. I have written articles on which catalogs are important for the gold coin specialist to own and won't be redundant be listing them again; use the search function on my website to look for these.
One other thing you need to decide is just how high-end you want your set to be. If you have deep pockets and lots of patience, you will probably want to purchase coins that are either the finest known or which rate high in the Condition Census. If you are have a more modest budget, you will want to stick with coins that are above-average for the issue but not necessarily in Uncirculated grades. Learning the quality of other specialized sets will give you a good idea what the best grades for each specific issue are.