Creating pages in php is great... you can really make life easier for yourself by breaking common elements (headers, footers, sidebars, etc.) into separate files and then telling the server to add them to each individual page automatically. That way if you want to add a link to your sidebar you only have to edit one file instead of one hundred. To do this 99% of the code is still ordinary HTML, only a small amount of PHP would be used.

CSS also makes things a lot easier because it helps you create a uniform style for all pages... again without having to edit every page when you make a change.

Designing a three column page without using tables can be challenging unless you fix the width of the page to a certain size. If you want the page to automatically stretch to fit the width of the visitors screen no matter what resolution it is set at then tables are the way to go.

You should be able to integrate the blog with the main site without too much trouble. I've played with Wordpress and the template system is pretty easy to work with.

Also, if you want all of the features of phpbb ver. 3.0 today then consider switching to SMF. I just switched my forum to SMF this week and I love it so far. Upgrading SMF is easier than upgrading phpbb, it's more secure, and it has more features. The only problem is that if you're almost out of webspace right now you probably won't be able to transfer the database.

As far as colour schemes etc. are concerned, I think the grey/white look your site has now is fine and I wouldn't change much. It's easy on the eyes.