The first thing I can recommend is to optimize your graphics. Here’s a few numbers: Your home page is 179k in size, which takes roughly 26 seconds to load on a 56k dial-up. Your “Benefits” page is 279k in size, which takes roughly 40 seconds to load. The bad news is that you’re probably losing a lot of potential customers who get antsy waiting; the good news is that those who are willing to sit through it really want what you have to offer.
I ran a couple of your graphics through Fireworks (there are a lot of programs out there you can use, including freeware) and there were some startling results from optimizing: your header graphic shrank from 71k to 20k; the other collage on your home page shrank from 65k to 21k; and your signature graphic shrank from an 11k jpg to a 2.5k gif. On your homepage that would eliminate 71k and reduce your load time by 10 seconds. You can run the math and see how much quicker that would make your whole site.
You might want to map out on paper how you want your site layout to look. On your navigation, your “Purchase” and “Calculators” links are identical with the exception of the dictionary in Calculators. You could save space and loading time if you consolidated.
There are two final things that I think are important to mention:
First: I, and a lot of the world have our monitors set to an 800x600 resolution. After you remove the pixels for the scrollbar and other browser chrome, there is about 760 pixels of viewable space. There are some people in the world who have rez set even lower. Your header graphic (and your nav menu) is 820 pixels wide, which makes seeing the whole page at once out of the reach of the majority of your potential end-users.
Finally: You might be on some thin legal ice by hotlinking to all the worksheets from calculator.com (all your pop-ups). I know as a webmaster, I would be upset to find that someone is using MY intellectual propety and bandwidth (even though DH gives me so much for little $) without duly compensating me. This may not be your case, but I thought it was worth mentioning.
The best place I found for putting some of these concepts (like optimization and site layout) in easily digestible nuggets is http://www.htmlgoodies.com.
That being said, I wish you luck!