Mutliplier is locked at 4.5. Can't change that.
In terms of speed, you should be able to hit 450MHz (4.5 x 100) without too much difficulty, and if you are lucky, you might be able to reach 504MHz (4.5 x 112). That's probably as far as you can go with the Celeron 300A, although you might be able to go a little further. It's highly doubtful that you would get up into the 550MHz area, even if you did increase the voltage.