Is this an OEM computer? Do you know who manufactured the board? You'd really have to know at least the model and manufacturers name before you could get any idea what the maximum supported CPU speeds are. There are freeware programs that will give you information about your motherboard and chipset, and that will help you find out what you can and cannot have with that mobo. One such program is Belarc Advisor, available for free download here:
http://www.belarc.com/free_download.html
Or you can download SiSoft Sandra, as that program also has tools to tell you whatcha got, along with benchmark tools. That program is available here:
http://download.com.com/3000-2086-10132058.html