I'm not being a total smartass but there should be a model # on the Compaq case somewhere. Sometimes they also put it on the system properties panel under the General tab.
If it's an OEM mobo you can screw things up if you grab a Bios from the Mobo manufacturers websight. But otherwise just use Sandra to find out what the chipsets are and try to match them up that way.
Beauty is in the eye of the beer holder.