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Description is very often used on SE results pages (google) so make it read good! #2 with a good description is better than #1 with just a senseless keywordlist as snipet. Put the keywords once, max twice into the Description.
Density as an absolut figure is probably the wrong aproach. It's rather the density of the usual use of this word in any text, to the density on your page, so best density is for every widget different. Check the density of your competition to get an idea.