matt,
It's a very open question. The answer is that it depends what the link to your ISP is and what services your providing to the rest of the world off your router.
Just as a guess looking at the infrastructure you have got I'd assume you aren't providing a www server or anything that takes a lot of BW. so I'd assume that you are pretty much ok with the 10Mbps link, I'd reckon the ISP link is probably slower than the ethernet service.
If you want to know what utilisation is, i'd suggest that you have a look at something like MRTG, which produces graphs and such for you to look at. It's free, and does a really good job.
HTH
Tony
