Well I have the 5500 but im sure they share some settings..here goes...
First off MAKE SURE You have it set to use 2X AGP, the default is 1.
Go to display properties,Settings tab, click advanced button, click the "3dfx Advanced Featurs" tab its under general.
ENABLE refresh optimization (disabled by default)
set yer anitaliasing..unless you are using a Cray you wont want to use anything over 2x
im not quite sure what "geometry assist" does..so i wont tell ya either way on that one.
Depending on yer system's speed you may wish to add a few notches of LOD bias... sa around .50-.75
Maximum buffered frames....im not sure, mine is set to 1 so...i dunno.
ENABLE mip map dithering (off by default)
Zbuff optimization, leave that on because...skuzzy said so.
as far as the OpenGl goes...I dont use it so...knock yer self out