Macs do no use BIOS.
There is no hidden settings menu.
The memory for the graphics card on all but a few iMacs are physical chips on the mother board, there is no way to add more.
As for being able to increse the available graphics memory on a PC, this is only doable if you have an intergrated graphics system, like the Macbook as these computers do not have seperate graphics memory, they use some of the main system ram.
The artical linked by italiano40 was last updated in 2000, so is really geared towards the old CRT G3 iMacs 🙂
So, in short, you stuck with the hardware you have, there is no way to increse the available graphics RAM.