I've recently been on a vlidating mission the past few months on my sites and I have seen a slight increase in traffic...but that could also be a result of other promotional activities too, so whether it actually results in more traffic, I'm not 100% sure, but it makes your site more accessible.
It's also like crack, it's addictive. Whenever I see one of my pages with the dreaded 'Red' page on W3 I feel dirty and need to clean it up until I see the heavenly 'Green' Validated screen. It forces you to learn more and become a better coder. I've learnt loads of little things to acheive that green goodness.
On your site -
http://validator.w3.org/check?uri=ht...pe=Inline&ss=1
Try adding this to your <head> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
You could try stepping up your DOC Type to - <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
Add alt="" to all your images.
Close your images - <img src="" border="0" alt="" />
Also, just one fix can sort out a number of issues, so a big number can easily be trimmed down. O'yeah, the Find and Replace function in Notepad will become your new best friend.
Be warned though, once you start validating you'll be hooked on it. But it does mean you can look down on those other, half-hearted, short-cut, dirty, filthy web designers.