check this link if you haven't heard about google heatmap, I just find it out recently. my site has doubled CTR by following the rules. https://www.google.com/support/adsense/bin/answer.py?answer=17954
I think you better consult the DP community by starting a poll to confirm the validity of their click volume heatmap. I'd like to see a heatmap of highest payout ad placement (not volume).
Top banner and left 160x600 for me and that bottom banner just above the footer really sucks on payout for all of my sites.
The link unit is very good. Place it in the navigation if you know that people will be clicking the nav menu often. If not, you should put across the top.