One of the appealing aspects to me of Safari 4 is Top Sites. This page is similar to the Speed Dial in Opera, only much fancier. I tried to find an entry in Safari 4 Help on this topic, couldn't find one, so I'm making up what follows based on my own experience.
Top Sites is a page in the browser that displays previews of pages from your history list (an assumption on my part that the history list is where these previews are generated from).
In the upper left corner of the browser is a symbol that looks like this: Click on that image to get a full page of images like the one on the left (but without the symbols in the upper left corner as shown in this image). In the lower left corner of that page you'll see an Edit button. Click on that button and then you will see those symbols in the upper left corner of the images.
Notice that the stick pin in this image is highlighted. Clicking on the stick pin toggles the highlight. When highlighted, that prevents that image from being replaced as you visit other sites. Clicking on the 'X' removes that image from the Top Sites, everything shifts left, and a new image is created in the lower right corner of the page - which will be the next page found in your history list. In the Edit mode, you can move the images around to be in the order that you like, and I'm assuming that the order is left to right from the top.
Items that are not pinned will be updated with previews of pages from your bookmark history, picking off the topmost sites as needed. (Another assumption on my part.)
In the lower right corner of Top Sites there is a button to select the size of the images. The smaller the size, the more images you see. The options are Small, Medium, and Large. With Small, you get 6 across and 4 deep. Medium gets you 4 x 3. Large gets you 3 x 2. The image size adjusts with the browser window size.
When you've finished editing the images, click on the Done button in the lower left corner.
Enjoy Safari 4!