Although, rather than going for high traffic general keywords, create many small adverts, with many many low traffic keywords. Optimise your keywords, take time doing it if you really want to reak the benefits.
I'll second MSN once you get past the clunky interface ( wish they'd support opera). I have some keywords on the 3rd page where as on adwords they would be buried on the 10th page and further down. Definitely less advertisers on MSN.
MSN conversion rates and cost/ROI factors weighed, on average they have passed Yahoo (formerly Overture) in my portfolio of inhouse and managed accounts. Ordered... G, M, Y, G-content. My good stuff all stops there. Note, no Y content.
Google is the only 1st tier PPC program at this time. MSN is a distant second thanks for the all their technical problems, Yahoo c'mon those clicks have been garbage for a long time now. And any of the other programs out there are a total waste of your money and time.