Well, all the people i know of view at least at 1024px wide screens. Thats the min. So 900 is good enough!
But still you can go till 800px, its better that you do not use a horizontal scrollbar. make the font smaller(maybe bold too to inc visibility), and break lines if u can.
It really depends on who the visitors are. If you want to be on the safe side, 750 pixels is the width you should use. Think of netbooks or people with older computers still using a 800x600 screen resolution. On the other hand, within the next couple of years (perhaps even less), 800x600 will dissappear completely. A width of 900 is not that bad. It will show okay on netbooks (which are most often 1024x600 screen resolutions) and it won't look so narrow on bigger screens.
Forgot to mention: horizontal scrolling really is bad. You don't want that. If your client says that most people can't view it without horizontal scrolling, I really would advise you to modify it and change it too 750. It may look verry narrow and strange on your own screen, but it may be far better for people who are actually using it.