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The amount of bandwidth required will be completely dependent on the characteristics of your application. You should have some idea of how many transactions per day you will have, the bps per transaction and the approximate correlation between the transactions. You need to use this information to approximate what your peak bps requirements will be and whether having some packet loss during the peak will be allowable.
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