I am putting my wife's K6-2/400 on a Micro-Star MS 5169 form a PCchips m598 which has an onboard video and sound.
She plays non demanding games, mostly the ones for the small grandchildern and scrabble. She does have Indipendence War and Shock.
These are the boards I am looking at.
ATI Rage Fury 32mb $90
Asus V 7100 GEforce 2 MX 32mb $114
Creative Labs 3D Blaster TNT2 32mb $77
Well, the GF2MX is the best card from that bunch, so would offer the best performance and also not be such a big bottleneck in the future if you ever upgrade. It is the most expensive, but not by far so I would get that.
Speed is everything - Warp factor 9, Mr. Sulu!