Welome to the Apple Discussions, FredericErk!
Pleased to have you here. 🙂
In answer to your question, OSX definitely is what you want to be playing with if you are planning on using your Mac to go online with. There really are no browsers still in support for OS9 other than iCab and many sites with Java will run poorly. It's more frustration than it's worth. Now if you are going to use your machine for burning vinyl LP's or graphics or non-internet stuff, sticking with the classic OS will suit you fine. There is more software available for OS 9 than you'd think It all depends on what you want to do.
OS 9 does have FireWire and USB support, though the latter is only 1.1. so it's slow. Apple downloads has driver support installation programs for both, so you do not have to go through full system re-installs to get the cards in and running. To get the high-speed USB you'll want to go to OSX however, and it will run well enough. I'm not sure of the FireWire sppeds since I don't have it.
The best version of OSX to install on a beige G3 is Panther, and you will need to use a program called XPostFacto 3.1 (
http://eshop.macsales.com) to make the installer load the OS on to that hardware. If your G3 is mostly original hardware, it will be very easy, and then you can add whatever upgrades as you see fit. To install the Panther you'd want to take the FireWire/USB card out first, then install the drivers and card afterwards. If it is the right
kind of card, you may not even need to add drivers, the OS itself has an astonishing array of support included. My Adaptec2906 SCSI card doesn't need drivers, they're in already!
As to how well OSX runs on a beige, right now, I'm in the middle of an upgrade failure, and my CPU card is being replaced, so I am running on the original G3/300 CPU and I'm having no problem at all - I installed the OS with it. The computer's a bit slow, compared to what the upgrade was, but this isn't permanent and it's not intolerable. You can check e-bay for sales on G4 ZIF upgrades, if you want a bit more speed.
Deb.