I shoot with a 60D, and Pixil Studio is right... Past ISO 800 the pictures aren't that great, and I would never go past ISO 1600.
2.8 should be fine, unless you're in the darkest 10% of churches... So unless you've been in the church, you'd be taking a bit of a chance.
We don't use flash during the ceremony ever.
As others have said, you might need a longer lens... About 10% of churches ask us to stay in the balcony... In which case you'd be screwed... Especially in a very dark church. *edit* Do you still have the 100mm macro 2.8?? We use it (IS version) at almost every wedding! If you are stuck in the balcony, you can use it to get some shoots, but you would really need a 70-200 2.8 to get the tight shots. (Ring exchange, etc)
Bottom line is, you'll learn more about what you need by talking with the officient and seeing the church in person then you'll ever get from us. 