You could force Safari to open these links in new tabs by Option-clicking on them, but if you’d prefer not to hold down the Option key all the time, you can make Safari open the links in new tabs by editing a hidden preference setting.  To do this, quit Safari, open Terminal (/Application/Utilities), and enter this command, followed by Return

defaults write com.apple.Safari TargetedClicksCreateTabs –bool true

When you relaunch Safari, links that would normally open new windows should open in new foreground tabs instead.  (To open tabs in the background, use the Option-click solution mentioned earlier)

To disable this hidden setting, quit Safari again, enter the following command in Terminal, and then press Return again

defaults write com.apple.Safari TargetedClicksCreateTabs

If you use Safari 4’s Top Sites feature, which displays sites you visit on an image wall.  If, however, you never use this feature, dump the images and use the following technique to prevent them from stacking up again.

To dump the images, choose Safari > Reset Safari and, in the window that appears, select only the ‘Remove All Webpage Preview Images’ option and click the Reset button.  To stop Safari from gathering more images, navigate to youruserfolder/Library/Caches/com.apple.Safari and select the Webpage Previews folder. Press Option+L.  In the resulting window, enable the Locked option.  Once the folder has been locked in this way, Safari will find it impossible to save images to it.