I read somewhere that the speakerphone and music over the speaker is quite good for a cell phone. I read between the lines there and understand that as the speakers being able to handle loud volumes at least decently well which would then mean to me that the ringtones would (or could) be loud as well.
Although, I will say based on my experience with my G1, that the loudness of the ringtone is VERY dependent on the ringtone itself! There are some that you would think would/could be loud but aren't while others are surprisingly loud!