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Personally unless you get type-ins from the names I would drop them all and just work with optimising the main site.
However if you want to keep them I would write a separate landing page for each one with your keywords in and a 'click here' for main site link. The single page sites wont however boost PR at all unless other people in turn are linking to them and it will be unlikely that any page will rank well in google just because there are keywords in the domain name. It used to work like that, however these days google will hardly ever use the fact that there are keywords in the domain to determine how it ranks, especially if its a new domain with little PR and not an established authority domain.
It is also possible that if you make too many 'doorways' you may get penalised by google.
Ta
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