First read the getting started blog posted on July 2, 2010, and if you follow the steps your account rep should be able to help you decide what are going to be the best coins to acquire for your portfolio. The reason you want help with this process is because every coin will perform differently in your portfolio. So you want to make sure that you are applying the right tool for the right job.
For example, if your investment horizon is short-term that should rule out any form of numismatic coins. In that case you will want gold bullion, which is a pure asset inflation hedge. Gold bullion provides the most amount of liquidity, because it is real money and it is accepted anywhere in the world. Gold bullion is used more as a safety net than it is as a growth mechanism in any portfolio. It is first and foremost a hedge against fiat currencies.
If you are interested in a long-term play then you can begin to focus on numismatics. In my opinion the $20 Liberties and $20 Saint Gaudens are the best option in this arena. They are the most affordable, liquid, popular and readily available option at this point in the trend cycle. Some issues have tens of thousands known to exist in a particular grade and some have only a few known to exist. The rarer the issue the more expensive the coin, but it will also have the most opportunity for growth. This is where strategy comes into play. If you are looking for asset protection you will want to focus more on common issue coins and lower grades, if you are looking for more growth you will want to focus more on rarer issues in higher grades (generally speaking).
A general disclaimer, I typically will only acquire coins in the mint state range, and always graded by PCGS or NGC. Grading by these two companies will add a layer of confidence knowing that you have coins that are guaranteed for their authenticity and level of preservation. As far as acquiring mint state range coins is concerned, if you look at performance charts on PCGS.com you will find that the mint state range has performed the best over time. If you have never purchased rare gold coins before I would not try to do it on your own, you need someone you can trust to lead you down the right path. Good luck!