I think you'll find that the AV option in the Beige G3s is not supported by Mac OS X... so if your main point of using that system is for video editing, Mac OS X is most likely not the way to go.
I just finished putting together a Beige G3 for a client. It has a 40 GB hard drive, 640 MB of RAM, I added the AV option plus maxed out the onboard VRAM to 6 MB (which is used in video capture). We are currently looking at either a 1 GHz or 1.1 GHz G3 upgrade as the software we are using (Premiere 5.1 and VideoShop 4.0) are pre-G4 software and would get nothing from Altivec. The system works nicely right now with the original G3/300 when capturing/editing 320x240 at 29.97 fps.
People today seem to forget that systems like these were being used for video editing in the mid-90s and seem to think that only high end systems of today can do such tasks... even though tasks like these were being done long before today's technology existed.
On the other hand, I'm finishing up a Beige G3 for myself running 10.3.9, 512 MB of RAM, an ATI Radeon 7000 (32 MB VRAM), 80 GB hard drive and (hopefully by the end of the week) a G4/500 upgrade that I'll use for small video editing projects using
HyperEngine AV. The 80 GB drive is connected to a ATA/66 card, the CD-ROM drive and DVD-ROM drives are on their own EIDE buses off the logic board. As I still have an open bay, I may throw in a CDRW in below the DVD-ROM.
I have a USB card in mine, but I could have dropped a USB 2.0 and/or Firewire card in it... but I got the USB card for free (as with the system... and all the parts I've put into it so far), and I haven't felt the need to spend anything on this system (beyond the processor upgrade) yet.
I have an SGI Indy that is designed for capturing video at full frame size at full frame rate, plus I just added a specialized video capture Nubus card to my 8100av (upgraded to a G3/500) that can capture at full frame size at full frame rate also (I also have Premiere 5.1 and VideoShop 4.0 along with QuickTime Pro 5 on that system). So I haven't had much need for getting my Beige G3 to capture video.
On an interesting side note... using it's original G3/266 processor, I have no problems watching DVDs in 10.3.9. It seems that the DVDPlayer software does everything via the graphics card and the main processor plays no part. QuickTime and some aspects of games don't seem to get as much help, but the system doesn't seem nearly as slow as one would think for a G3/266.
Still, it doesn't take too much to tax it to the point of realizing I need that faster processor.