If you click "Advanced Search" underneath the search field, you'll have access to various search filters. Be sure to select "Search Titles Only" from the drop-down menu next to the "Keyword(s)" field to help get more relevant results to topical searches. Unfortunately, I think if you use multiple keywords, there's no way to specify that you want only results that contain both keywords. For example if you enter the keywords imac ram, the search results (for thread titles) contain a lot of threads with "iMac" in the title, but not "RAM".
The Google search as Ashwin described is great for looking for specific phrases on the forum. You can also filter the results by time by clicking on Tools (under Google search bar) and then the drop-down menu that is defaulted to "Any Time" in order to change that parameter.