PPC advertising is getting more like SEO now because Google, for example, does not place your advert in the top position just because you placed the highest bid, instead it uses something called “Quality Score” to rank adverts. To get a high quality score you need to bid well, make sure your advert text, headline and landing pages are all relevant to the serach query. This is where a site that is well optimised (SEO’d) will work well for PPC too.
In terms of which provider to use, Yahoo is cheaper than Google as there are less people using Yahoo Search marketing. Although you will get quite a good number of advert impressions in Yahoo, Google will always drive more traffic because it is still the most popular search engine (in the UK anyway). Unfortunately, I have had very little traffic from MSN Ad Centre.

The good thing about PPC is that you can get results quickly and they are easy to track and measure.

Yes it works; in fact, it works better than any form of advertising in history. Why do you think Google is $500+ a share? My advice is hire a company with over 6 years of experience and who is certified. In terms of affiliate marketing, you missed the boat, it is too late unless you have a large budget and a big support team.