If you have to ask, you will probably want someone there who knows what they are doing.

But yes, you can just replace the rings if you wish. However that is not recommended. You cannot replace the main journal bearings with the motor in the car because the crank will be bolted to the flywheel, which is connected to the clutch, which is connected to the input shaft on your tranny, so u cant drop the crank. And if you took the effort to drop the tranny, you should just yank the whole motor because it will be easier. If your rings are worn, you will probably want a bore/hone (which may require new pistons). It all boils down to how much you like your car I guess. I would just replace everything while I am in there, because my labor is worth more to me than saving a few hundred bucks on a rebuild. Rod bearings can be replaced with the motor in the car tho