Hey, welcome, Mike. The BB is made for guys like you.
As for ringtones, yes, if you use a ringtone that's encoded with a higher volume, it will be louder when it rings.
Treos are famous for needing constant freezing/rebooting. BBs are much more stable especially if you're careful about aftermarket software. This is a non-issue.
Tethering is definitely possible. Most other carriers don't charge for that. Some of your tethering needs will be fulfilled by the internet capabilities of the device.
I don't have 88xx experience, so can't speak to the issues. I can say that RIM, the manufacturer, has been very proactive in issuing software and firmware fixes. This platform is still a bit new and some kinks are still being worked out. Carriers, however, don't always release the upgrades so quickly. If you stay tuned in here at the forum, you'll be able to find the latest upgrades even if your carrier is slow.
Must have software: google maps, opera mini browser.
8300 Curve: viigo, gmail, google maps, way too many emails/day