Hello Dave,
There is no documentation for specific tables. Most of the imports I have seen users do is with customers and leads. I do see some for activities and opportunities. There is a common process to do this to do any imports.
1. First find out what fields are mandatory. Go to Customize > Tables/Fields > Opportunity to open the field list for the table. Look for any fields that have "Required?" checked. In opportunities AssignedTo, CloseDate, Customer, Name and OpportunityStage fields are required. This means they must be in your import file.
Three are dropdowns. AssignedTo is the dropdown connecting to the Users table on the spdSecurityUserName field. (You most likely can use just the user's display name) Customer is the dropdown connecting the Customer table on the FullName field. OpportunityStage is the dropdown field connecting the OpportunityStage table on the OpportunityStage field. When importing, these three fields must have values that match values in its respective field and table. For example, if Customer has a value of Sam Crenshaw Fine Foods, 'Sam Crenshaw Fine Foods' must be in one Customer record in the table that has FullName field equal to that name.
2. Export the opportunities table and create your import template. This will give you a base for the opportunity import. I suggest creating one or two dummy opportunities before exporting. It will give you markers as to what fields values are stored into, in relation to the screen fields.
3. Create your import file just like you would do with importing customers remembering to include the required fields. I like to save a blank template, a template with all your data and then your actual import.
4. Import the file. Try two records the first time. See if the imported records have the correct data once in Method and that the links work and you can see the record. If something doesn't look right, then export the table again and take a look to remedy the problem. I recommend doing only 500 at a time. After 500, you run may run into timeouts if the import is large.
Greg