3 things...

1. Is your 1400c the 117MHz or the second-rev 133 MHz model?
2. It's a good thing your memory is maxed. If you priced that memory today, you'd be shocked at what it costs.
3. So you have 2 1GB partitions? My 1400c only had a 2GB drive in it.

I don't know what the storage limitation would be with the Sonnet card? Do they make ATA-133 drives bigger than 250GB right now? I'm using only 1 partition of 250GB and it works fine in OS 9 and OS X. Just isn't bootable... or I haven't managed to boot it yet anyway. It seems to wake up the Sonnet card after the POST routine has completed, so I don't think it will. It can run a master/slave/master/slave, so if you maxed it out with 4 250GB drives, you'd have about a Terrabyte in a Blue & White case. Not bad. I don't like running slaves, though. I'd use 2 Tempo ATA133 cards to do that.

Edit: The limitation should be whatever 32-bit addressing maxes out at for a single volume. The 133GB ceiling is caused by the limitations of 24-bit addressing.