Hello Adam,
It's great to hear from you too.

It's been a long time since I frequented the forum but I still pop by from time to time.
I just went to Yamaha's website and looked at the comparison table between those two models. I'm guessing that you've done that already, but if not, it's an easy way to see some of the features side-by-side between the two.
Regarding the power, I wouldn't pick one over the other just based on the spec numbers. 90W versus 80W isn't that huge. Rather, I'd look at some of the features. For example, the 673 is 7.2 (mono sub outs, though) versus 5.1 on the 473. The 673 will also convert analog video inputs to HDMI out, which might come in handy depending on their equipment.
Yes, I have compared the specification for all RX-Vx73 models and came to the following conclusions (relevant to my parents situation/my personal preferences):
The '373 has the least power, spring clip speaker terminals and does not support control of the reciver via iPad (only iPod/iPhone) so that has been ruled out.
The '573 offers the same power as the '473 but into seven channels. My parents will never setup surround backs - they may not even setup surround channels (deterred by too many speakers in the room/cables running around the room) hence my previous mention of a 3.1 setup. So the '573 has been ruled out.
The '773 offers only marginally more power than the '673 and in my view would unquestionably be overkill for my parents, given the increased cost over the '673. Hence that has been ruled out.
Hence the choice between the '473 and '673. I should also state that the speakers will be reasonably sensitive (85dB/W/m or greater), so I don't expect those to tax the receiver, regardless of the relieving effect of a future sub.
I am aware of the '673 having dual sub terminals but, as you say, they are mono. My understanding is that there is negligible difference between this and splitting a single sub terminal into two - my Z9 has dual sub terminals (can't remember if they are mono or stereo) and I intend to use a splitter for each in order to send content to four (in order to help control room acoustics) subs.
Upconversion of analogue is not a feature my parents are likely to require - they recently purchased a plasma TV and will replace their DVD player with a Blu-Ray player within the year, so signal transfer will take place over HDMI. The DVD/Blu-Ray player will be used as the CD player and transfer over coaxial/optical SPDIF (so that my parents can select Blu-Ray/CD input separately on the receiver), so again, exclusively digital transfer of information.
My gut tells me that the '473 will be fine, but I have no feel for the power capability of an 80W receiver - my Z9 has more than double/almost double the power of the '473/'673 respectively. I actually am surprised at just how little power the flagship '773 has by comparison, which serves to remind me just how astonishingly good the Z9 is (okay it's not a fair comparison given the price differential, but still).
I've hooked up my Z9 to my parents speakers (Monitor Audio GR10 bookshelfs) in order to test them (purchased second hand in order to maximise the quality of equipment they get for, by my standards, a very limited budget) but of course the Z9 drives them effortlessly (the GR10's are really lovely sounding by the way), so there is little to be gleaned from that.
Any other thoughts are very welcome