Welcome to WebmasterWorld Guest from 22.214.171.124
The top-5 words will probably have a higher CPC, resulting in a lower ROI. However, they will be easier to manage -- resulting in a higher ROI.
On the other hand, picking 600 other words will probably result in cheaper CPC (that ought to be the goal, right?) and higher ROI. But it may take much longer to manage, and that will hurt the ROI.
So for your situation, it will just depend on the cost of managing and how much you can optimize.
For my part -- 500 words is but a small step in the right direction. :o) Automation, process, and lots and lots of keywords is the scalabale way to go.