encyclo - 5:29 pm on Jun 13, 2006 (gmt 0)

Having valid markup will not in itself help ranking - but conversely invalid markup can have a negative effect.

Perhaps you should really ask the question about how many errors you can allow in your HTML you can have before it becomes a problem? For example, mis-matched (incorrectly-nested) tags can lead to sections of text being ignored by the spider.