The string "SIS741GX-MN" brought up a bit over 800+ hits on the web, so not too much to go on. Seem like some kind of bulk motherboard with no definite brand, but likely its origins from PCcchips/ECS line.
# Supported Processors:
# AMD Athlon XP/Sempron/Duron
# Socket A processors
# 200/266/333 MHz FSB
So likely it tops out at Athlon XP3000+ /333MHz and Sempron 3000+ socketA/462.
Win98se is not a limiting factor in this matter.
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