usually the PC133 will step down to PC100.
You will not see a big jump in speed with more memory, but some small improvement.
Not sure why it is slow in browser mode. That sounds strange. You might consider dropping back to OS 9.2.2
http://docs.info.apple.com/article.html?artnum=58418
You have the Power Mac G4 (Gigabit Ethernet)
Although 2 GB can be physically installed, a maximum of 1.5 GB is recognized by Mac OS 9. Mac OS X will see the entire 2 GB.