Theoretically you can still here the uncompressed audio, but if you don't have an HDTV I'm going to assume you don't have decent speakers to hear the audio either.
I have a Sony A/V control center that the audio goes through to my "box" speakers, which are old but of reasonable quality, since they're used for my audio system as well. Because my TV doesn't have good speakers of its own, I generally just use it as a monitor for the cable signal that goes into my VCR.
Anyway, my questions about rental may have been moot. My local Blockbuster is closing (because the adjacent grocery store bought the lot, not because of the company's overall problems, purportedly) and isn't getting new videos anymore, and the next-closest one is too far to be worth driving to with gas prices as steep as they currently are.