hmm 98SE isn't one of the best OSes out there, 2000 is far more reliable for most heavy load situations.
anyways a Gigabyte mobo with Intel chipset, uhm Intel I850? I865? I845P? there are so many of them.
an Intel chipset may be the issue since my AthlonXP with ASUS A7V333-R Rev 1.02 + Beta Bios 1017 VIA Apollo KT333 chipset and really it never failed under 98SE, 2KPro & XP Pro with a 9700 pro 9800Pro and X800XT PE AGP.
though Gigabyte like ASUS, ABiT and MSI are good brands overall.
So it might be a problem with your intel chipset and ATi card, which may sound silly and yeah it should work really.
uhm did you flash your mobo with a the latest bios that may help? you can also flash your ATi card with a newer bios that can fix things too
It's so that support for 98SE has stopped for quite some time, so support for that OS isn't important which also maybe the reason why ATi advises a later OS.
since it functions perfectly under 2K pro
really XP pro has great advantages , you can run 98SE games as programes unsder 98SE compabillity mode, and if that don't cope there is always DosBox
but yeah 98SE can run stable when you install it on a stable system, my ASUS A7V333-R ran 98SE with the 9700 Pro and 5500 PCI for 2 years without lockups.
I personally think it's the intel chipset problem with the ATi card under 98SE, just a bad support, could also give gigabyte the blame for bad support under 98SE with ATi cards
maybe a newer chipset driver might help, look elsewhere