There are formulas that can give you the exact kind of answer you want. Take a look at the traffic category at http://netbula.com/anyboard/guide.html
As far as 5 gb of traffic being enough, it depends on the size of the pages (graphics included) and how much traffic your site gets, and other things, like downloads. I can see where it might surpass 5 gbs at some point (if it doesn't already), although 5 gbs is much more than the majority of sites would use.
You need to be aware of any host's charges and plans for when you exceed the alloted bandwidth. Some will simply charge you for the excess amount, and there can be a big difference in what they charge. While bandwidth is a slow as $3 per gigabyte, some charge $20 per, albeit in increments of 100 mb. Others might disable your account.
Some hosts have upgrade plans, especially if you regularly exceed the alloted amount, though many amount to no more than increasing your charges by what they charge for excess bandwidth, with maybe a couple of dollars off.
One of my newest accounts came across such a plan at one of the hosts he was considering. The plan he was considering had 6 gb, excess bandwith was $2 per 100 mb ($20 per gb), and "upgrades" were $18 more monthly for another gig of bandwidth, and I think a slight increase in disk space (which is so cheap as to be inconsequential).
A separate issue from bandwidth is that of resources (cpu cycles and such). Find out what happens when you exceed whatever a host considers a fair amount. Do they have an upgrade plan, do they shut down your account (many do), disable cgi programs or others that consume large amounts of resources?
Your questions aren't stupid. They just indicate soemthing many of us share, ignorance in the ways of web hosts (ignorance is very different from stupidity). It's better to ask before you commit to a host and get stuck with a bad one. There are a lot of them out there.
Some of us (myself included) have learned ad much as we have out of necessity. After being burned by one or more bad hosts, you start researching to make sure it doesn't happen again. That's the one positive thing that came out of the lousy service and experiences I've had with only 2 bad hosts, I've learned a great deal (and have much more to learn).
One bit of advice: don't be dazzled by the technical offerings of any company. They mean nothing when customers are neglected, ignored, and abused.
[This message has been edited by Duster (edited 04-18-2000).]