It clocks up when your GPU is under load, and clocks down when you aren't doing anything.
This saves power and gives you performance when you need it.
800mhz versus the 540m stock 672 will make a big difference i hope just got the lenovo G870 open box for $470 with i5 ivy bridge. cant wait have you used the 630m? how is the performance?
i have a desktop if i want 1080p etc but looking at videos and having a gt 525m before i can tell this will perform much better. it all depends what your definition of low end is ;p