Ok, here's what I've decided upon. I paid in full for the RockHopper Comp Disc ($650 w/ Hayes Nines), and ordered the HardRock Pro Disc for my son ($550). I have a tendency to buy more expensive things than I really need, and this seems to be no exception. But my general philosophy is that you get what you pay for, and as long as I am getting excellent quality for my dollar, I'm happy with spending a little extra. Besides, if for some reason our new hobby/sport doesn't take, we will have a couple of higher value bikes with better resaleability!
Regardless, my son and I are both very excited about these bikes, and we can't wait to get started. We found a Labor Day sale at REI where they have Bell Furio helmets on sale from $60 down to $44, so we plan on picking up a couple of those, and maybe some riding gloves (any suggestions?).
As for riding shorts - I saw some boxer-type things you wear under normal shorts. Are these any good? Do they offer rash protection the same as full-on riding shorts, or is there a better way to go? Further, do we even need shorts at this point, or should we hold off a month or so?
Lastly, let me thanks every one of you for your insight. I am a Moderator on a BlackBerry forum, and I deal with newbies all the time, and sometimes it gets frustrating answeriung the same easy questions, as I'm sure you guys know. But you have all been patient and very helpful with my ignorance, so thanks a bunch.