Incredibly generous, very thankful! I do have an install of Mac OS X 10.3 on it too even though I'm aware it's just above the minimum.
Latest update:
@Phipli very kindly provided me with another 192MB of RAM on a 128MB and 64MB pair of sticks. I already had 3 DIMMs used (64MB*2+32MB), so I removed the 32MB and added the other 2. I also used one of his 2mm jumpers to replace my hacky use of the original jumper block.
However when I rebooted, it wouldn't complete the boot - it got stuck on Mac OS 9.1 loading up extensions. Then it wouldn't even get that far on a restart. So, that was either the RAM or jumper settings at 400MHz. I switched it back down to 350MHz and it fully booted to Mac OS 9.1 with 320MB of RAM

Gosh, that's serious dude stuff for 1999!
Then I rebooted into Mac OS X 10.3.9. There's now plenty of RAM free, even when I run iMovie 3.0.3 (which seems to be taking up 34MB). However, iMovie 3.03 supposedly needs a 700MHz G3 - anyway, my HD bus is too slow to use it until I get SATA sorted.
I say 320MB was serious dude stuff for then, but it was true. Most Macs were shipped with 32MB at the low end (iBook/300, iMac/266) or 64MB at the professional end (PowerMac G3 and even the low-end PowerMac G4s). Only a couple of Macs were shipped with 128MB. In 1999 I had a PB5300 (which I keep going on about in an attempt to deal with the trauma) but that had 40MB. When I bought my Tangerine iBook/300 in spring 2000 as they were going cheap before an imminent update, it came with 32MB of RAM so I immediately added another 64MB.
One of my goals with my B&W G3 is to do some video editing using my Canon MV550 (which still works and I have tapes!). I plan to install EditDV 2.0 (or 1.6), which I figure can't be too different to FCE 2 and the system requirements are much lower (I think, though I can't find a reference for that).

Though having said that, EditDV runs on Mac OS 9 and I'm pretty sure my sister used iMovie 3.0.3 under Mac OS X on her iMac 400DV and I used it on my iBook/600. Theoretically they're not too different in performance:
| Model | RAM/MB | MHz/L2 Cache | Geekbench2 | Ratio |
|---|
| B&W G3 | 320 | 350/1MB | 210 | 100% |
| iMac 400DV | 320? | 400/512kB | 211 | 100% |
| iBook Clam (FW) | | 366/256kB | 178 | 85% |
| iBook Clam SE (FW) | | 466/256kB | 216 | 103% |
| iceBook dual USB (FW) | | 500/256kB | 214 | 102% |
| iceBook dual USB (FW) | 384 | 600/256kB | 290 | 138% |
None of them are hugely superior (I threw in the Clamshell FW iBooks as they were the slowest FW iBooks). My iBook started with 128MB+256MB, so again, similar to the B&W. So, yeah I think the system requirements for iMovie 3.0.3 can't be right.