I have to say, changing the width to 72px fixes it for me in both my browsers, both monitors and in all zoom levels with assorted Windows text sizes. However, I do understand that YMMV as we say on the Internet.
If by "page size" you mean the width of #container as 960px then I don't think that is the main issue because if you comment that out it just moves the problem to the far right of the window. (Maybe #container should really be a class because it is used more than once?)
That group of 4 items also has a fixed width of 310px (and a margin of 70px) so after the width of the sign up button has been fixed then that might need changing as well.
I note that the button backgrounds do not keep size with the text on them, either. Basically, there are lots of fixed widths that could do with some relaxing IMHO.
Now I must go and tell them about my organ playing in the web design forum.

Regards,