I second and third the previous comments: more ram, faster HD. Or just save yourself and get the macbook.
from a 1.07 to a 1.2? You're not going to notice anything like the jump in speed you'd get from just upgrading to a new machine, and you're looking at two costs here.
1: the cost of the upgraded card [which may not be reliable even after you've put it in]
2: your time and energy. I have an ibook that's a pet project, I've opened the case and replaced hardware in it several times. But it is a seriously agonizing project each time. Those machines are not intended to just be taken apart and futzed around with. You're talking several hours of work just to get it in, then several hours to rebuild your comp and test everything to make sure it works. And if you miss a screw or forget to plug in the sleeplight [like I did], you get to start all over.