from the manual:
First thing - don't change plugins! Resist that urge.
Don't select "T&L HAL". Or anything else. Leave it on Direct3D HAL.
Don't enable "Sync game to Audio". It is bad mmmkay. Don't touch the RSP configuration at all
Don't touch graphics "Advanced" tab at all
definately don't touch the "ROM Settings" tab, horrible things will happen and you'll get all confused in there
don't start using Cheat codes staight away either.
yeah that was a bit too sarcastic wasnt it, ive toned that down in the updated manual, but to answer the Q - that is in video plugin not RSP plugin and is autodetected on boot/state load - theres no point changing it yourself since your setting will be overriden, its there in case autodetect fails (most people will never see it fail).