Ham's prolly your best man, but as far as insulation goes, I've heard quite a few good things about rubber paint like this
. Obviously nice and easy to spread on. If you get a black one it wont stand out too much on the motherboard and best of all if you ever need to rma the board you can just peel it off (if you paint it on thick enough).
Dielectric grease makes an awful mess and can be a RIGHT PITA to clean up.