Site Redesign 75kb Tables/Bad HTML -> 33kb CSS/XHTML

[MrLister]

Ok well I have a site that receives about 70,000 users from google every day. To help increase ranking and beat our competitors we've decided to turn the site into a no-table XHTML/CSS website. I have the final product but before I put it up a few things I'm concerned about:

The HTML code is drastic and the two source codes don't even look the same one bit but the content, links, title, alts, description is all the same.

A) What kind of impact will this drastic change have on google?
B) Do you think switching over to XHTML/CSS and W3C valid is a good idea?
C) A lot of people when they see <div>'s and CSS they think hidden text and hidden keywords, how can I ensure this doesn't happen / coder doesn't screw up / google doesn't think I have hidden text?

Offtopic D) Anybody know of any sites other then sitepoint that hold competitions for contest layouts?

Thanks