Here's my bit of proof why it's better. Last year i finished a site that was plain old html 4.0, and, it didn't even validate. I had a doctype, i had keywords, robots txt., i had good content and titles. Basically, i had everything that seemed to be "a good lunch" for search engines. I submitted to google and yahoo in the same day. I was listed in google about a month later, and Yahoo...it took about 4 months. I only submitted once.

In contrast. I spent the past year focusing on the "tiny" differences between html and (x)html because i suddenly became interested in "standards and validation." Since February, i have finished two sites, both valid (x)html, and submitted them both (about a month apart.) The first website was listed in google within a week and half and in yahoo in less than a month. My site was listed even faster. I had almost simultaneous inclusion in both yahoo and google within a week. Soon after, i was included in msn, dogpile, askjeeves, etc. Again... i only submitted both pages once.

I think the "valid" code does play a part in this, but code can still validate and be a complete mess. So much so, that spiders and bots will not consider it.

There's alot to consider. Keywords and relation to body text. Page titles and consistencies. And, of course links and content. But, in my experience, i would say that valid (x)html teamed with valid css can make for a very enticing morsel for search engines to feast on... the first time they find it.