To preface my comments, I'm just finishing up a year long school program on Web Development... basically I'm becoming a professional web site maker. And I do have a few suggestions to make here:
1. The site is too cluttered, divide it into multiple pages, have one for news, one for your products if you're selling anything, one for your sponsors, one for your forums, and one for your blog.
2. Advertisements on every page is just fine, but rotate them and put fewer of them on each page. With the sheer amount of links and images you have in the right colum my eyes start to glaze over a bit by the time I get to the bottom. If you have them changing a bit then it is refreshing and different and will catch the average user's eye more.
3. Make the page as wide as your banner at the top. The white space on either side is fine as long as the margins stay consistent with it, particularly on the left side of the page.
4. Make your banner wider, joebabyny is right about that. If it gets too tall when you make it wider, see if you can get it redone to the proper width.
5. Make the colours of the rest of your site reflect the ones in the banner. You have a fantastic colour scheme in your banner, and they are not repeated at any point in your site. Setting the forum colour scheme to reflect this would work wonders for you. Also, don't be afraid to set backgrounds to other colours than white too.
Personally, the forums I like best are Simple Machines forums, but having never used proboards I can't say how good or bad they are.
I like how your navigation is set up, just make sure if you do multiple pages to keep it consistent across all of them.
In regards to saving images as jpegs, every time you do that, it actually compresses the image and throws away data, thus making the image a poorer quality. Personally, I like to use PNG images.
I hope this helps,
Barm