Some html validation errors will screw up page layout, others will not. For example, consider
echo '<select>';
for ($i = 0; $i < 10; ++$i)
echo '<option>' . $i;
echo '</select>';
This will display a select box with 10 options, and also generate 10 errors since each option lacks its closing tag </option>.
However, trying to find out what breaks display and/or functionality or not due to validation errors, isn't something I'm willing to do for someone else, and most people feel the way I do.
Moreover, if you ever need to make changes to a page that used to work, and it suddenly breaks, things are easy if your paged used to validate fine. That way you can for example easily find out that you have a missing closing tag for a div. If you have 500 errors on the page due to images missing alt attributes, options lacking closing tags etc, you're in for a lot of time tracking the actual problem down.
And while it may be a little bit of work to get your first few pages to validate, it will get easy with time. After a while you stop thinking about what's needed and just automatically write correct code, at least for the most part. Then you run it through the validator and fix the last few errors that snuck through.
In either case, I tried your code, but since I don't have access to your included files, I just replaced them with plain text "MENU", "CONTENT" and "FOOTER". The right bar did show up, but it showed up below the rest of the stuff. However, since it has no border and contains nothing, you won't actually see it.
<td class="RightSide" width="200">
</td>
Perhaps it's supposed to be dealt with by css, but you did not post any css. Also, if you're supposed to deal with it through css, why use tables instead of divs?
I'm guessing what you want is equal height columns such as http://www.ejeliot.com/blog/61
