70-300mm IS USM

I've decided to purchase the lens 70-300mm IS USM. I can find it online for a good price and it seems like the best choice for the telephoto zoom lenses.

My question is regarding the aperture. It's rated f/4-5-6.

Here is a scenario;

I am in a cathedral that has very high vaulted ceilings. I would like to take a close up image of the detail on the ceiling but this is indoors with restricted lighting. My first thought would be to reduce the shutter speed and set the ISO to 800.

I know that this is a hypothetical situation but how would this lens act in this environment regarding exposure? Past experiences would help.

Thanks