Pretty easy to tell. If you are running 10.3, open Activity Monitor and click on the System Memory button (If you are on 10.2 or less, open terminal and type "Top"). Look for a section that shows the number of page-ins and page-outs. If you have a large number of pageouts, you're system will benefit from increasing the RAM. If not (or if you have zero pageouts), then going from 512 to 768 will not increase your system performance. Try checking it when you have Dreamweaver + lots of other apps open. You can visibly see the pageouts changing if you don't have enough memory to handle everything at once.