Another consideration you'll need to make is whether you will be able to segregate your important traffic from other non-critical traffic (employees web browsing, e-mail, etc). The non-critical traffic could have a significant impact on the critical traffic (example, Web browsing uses 6 Mbps of the 8 Mbps). One way to fix this is to prioritize your really critical traffic.
With not much information, I would guess that 8Mbps for a brand-new business is actually going to be a lot. If you have very little information to estimate actual traffic loads, you would definitely want to buy a burstable service with a lower minimum threshold so that you can grow your commitment to your ISP as your business grows without incurring a large up-front monthly cost (and later potentially realizing that you don't need that much bandwidth.)
This was first published in June 2003