My G4/400/AGP with a Sonnet 1GHz upgrade runs absolutely fine in Panther. You can also be pretty sure that the upgrade companies will work overtime to resolve any problems should they arise - after all, making things work well in the newest operating system is what the upgrades are all about. If Panther didn't work on these upgrades, nobody would buy 'em! Apple says that they don't support Panther on the upgrade cards because Apple didn't manufacture the cards - nor do they test them. And, as Scott pointed out, it isn't in Apple's interests to support these upgrades! Apple is a hardware company, after all.
