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There have been several discussions about this topic, which you can find if you browse around the forum.
These stats change very often, so it's difficult to make an assertion to answer your question. Currently, I believe MSN was rumored to be receiving more traffic that Yahoo, followed closely by Google and and AOL. I can't find the link to where I found these stats, but perhaps someone will come along to correct me.
I presume you're gathering this information in order to optimize your site/s. The amount of traffic you receive will depend on what KW you're ranked for, your user demographic, and where you're ranked.
Hope this helps! In case you're wondering though, Yahoo does have the potential to be a great source of traffic if you get a good listing.