There are two ways to "invest" in gold.
1) Buy and hold the actual metal. You can buy coins, bars, wafers, whatever. This provides a nice touchy sort of investment, since you can touch and hold these. There is a feeling of security. However, these are NOT the best way to cash in on gold. If gold is selling at $400 an ounce, you will never be able to buy a 1 ounce gold bar for $400. You are going to pay a premium. Depending on where and what you buy, you'll pay $420 and more...Start buying 1/2 anmd 1/4 ounces and the price starts climbing dramatically. In addition, its hard enough to get rid of. Where are you going to cash it it? You are not going to go to the grocery store and slap it on the counter... Gold, although it has great value, is a real pain in the ass.
2) Buy gold "paper". Invest in companies that produce and hold gold. As gold prices go up, their stock value goes up. These are paper investments, and do not have the touchy feely benefit. However, if you want to buy more, you log onto to brokerage account and place the trade. If you want to sell its the same deal. Nice quick and easy.
Here is two I own:
GoldCorp. Gold mining company. It has some of the lowest per-ounce production costs of any mines in the world. When gold is selling at $400+ and you are "making" if for close to $200, and you hold a ton of it in reserves, its a nice place to be.
Toqueville Gold. Mutual Fund that invests in gold and gold producing stocks.
While the gold paper(stocks and mutuals) do not have teh hard intrinsic value of a bar of gold, these can be bought and sold with ease.