Welcome to WebmasterWorld, cantwont!
I'm running between tasks at the moment, and am not able to post in depth. But I did want to at least make a start on an answer to your question.
In my opinion, a high number of impressions is not really a goal that one should shoot for, simply for it's own sake. Rather, it is the quality of the impressions that matters. And by that, bottom line, I mean their likelihood of bringing you an actual customer.
To put it another way, a thousand impressions that bring you 50 'window shopping' clicks, but zero sales, are worth far less than 100 impressions that bring you 5 clicks from really qualified customers, that then lead to two sales.
So (and again this is just my opinion) I'd think beyond simple numbers - and instead choose groups of very specific keywords that are all highly targeted to what you have to offer. And then have those keywords bring up an ad about exactly the same thing.
Hope that makes sense, and that others will add their thoughts as well.