W3C Validation - how important is it?

[leefish]

I'm a noob at HTML (and css and php) and I am having real problems getting my HTML to validate. I make the page, it looks good, I check the pages in Opera, Firefox, IE6, Safari on my own PC and use web tools to check my work.

It all looks fine, then I take my page to Validator - and it does not validate. I don't mean things like missing alt and things - on one page the validator is reporting over 300 errors - things like closing tags - but they are there. It looks fine until I try and fix what the validator says is wrong then my page is broken to hell. Should I take the validator results seriously?

link to my page:

http://www.leefish.nl/mybb/forumdisplay.php?fid=16

I know the header is a bit shaky but the rest of the HTML looks ok (as far as I can see). The reason I am asking is because I have made an Xthreads example, and I would like to post it here - but not with crap HTML