Hi MrsLop--and a belated welcome to TS. I used a 6115 for years and my father still does, with his 56k modem. He uses Mac OS 8.1, and this has helped a fair amount after using OS 7.5.
I agree with Spartacus. If you went high speed, the information might GET to your COMPUTER fast, but your computer wouldn't be able to get the information to YOU very fast because of the speed of the machine, amount of memory, etc. It's like throwing a book like "War and Peace" at me. It would get to me in a hurry, but my brain still needs to process the information once I start to read it. (And as the years go on, this process seems to be taking me longer and longer.
)
If you're going to stick with this Mac, I'd use a 56k modem. Another thing to keep in mind to save money--don't be so quick to spend $21.95 or whatever a month for your internet provider. There are others that are cheaper and may be available where you live. Can you tell us what region you live in? For example, my father uses an Internet Service Provider that I also used until I changed to DSL in October. By prepaying fora 6 month period, he pays $11.95/month. Even without prepay, it would only cost about $14.95/month. This service services many states in the Northeast area of the country. It's called Galaxy Internet Services:
http://www.gis.net There are other providers with similar rates that may be available to you.