I recall seeing stats that said that for similar positions in the
search engines for every click you get from PPC you get x2 clicks from
a natural position. And if you have both positions you get x6 clicks.
Obviously these stats would vary widely depending on circumstances,
industry, etc. However I can verify that when starting a PPC campaign,
your SEO traffic will increase without any SEO effort what so ever. I
have seen this happen many times.
This synergy makes sense. By taking 2 positons you are just doing what
traditional advertisers know is important--repeating you message. In
addition you are crowding out competitors.
I personally feel more confident about clicking on a natural search
engine result if there is an ad off to the right. That shows me that
the site is still being maintained since the owner is paying money to
drive traffic.

One last point. I find this metaphor helpful when explaining the
difference between PPC and SEO to potential clients:
PPC is similar to advertising. You have control over the message, when
and where it will appear and more predictable results.
SEO is similar to public relations. The impact can be much greater but
you have less control as the search engine (or newpaper/TV station)
decides what, where and when something gets in. Even though SEO is
less predictable, and the return takes longer, it almost always has a
better ROI.

Ron