I love to make web sites W3C validated. I think it's professional. A W3C validated web site is more viewable and accessible than others. In your opinion the W3C validation can influence the positioning?
Yep, it is one of the factors to achieve a top position. It is not a crucial factor but in a competitive market it can be the difference.
It's actually not a factor, and it won't help you rank better. It can however make it easier for the spiders to crawl your pages, and it'll also help ensure that your pages work better in just about any browser made since 2000 (I've gotten to the point where I can get rather complex layouts working from IE 5.x to 7, Firefox, Opera and Safari without using any hacks, other than the ones that are used to provide equivilent functionality, such as min/max-width/height, PNG alpha transparency support, :hover/:focus, opacity, and so on).
Got a site ranks #1 for major keywords, its an old one, all tables. I can code a new one tableless, valid W3C, but the idea of changing it scares me to death... So, don't know, maybe in a more competitive niche a valid one can make the difference!
It's not really important. Take a look at those first page listings on search engines, how many are W3C validated?
i have seen sites with top ranking in most competitive industries that returns validation error. that tells its not important as much as it seems but its good to have a site that passes the validation