The two things you'll need to look out for are reach, and the type of bolt hole. Make sure the brakes you buy aren't the "recessed" type, unless you want to take a drill to your bike!
Reach is measured from the middle of the bolt hole, to the middle of the braking surface on the rim. Or if your brakes that are on there right now are weinmann, it's the model name stamped on it. So Weinmann 500 would be 50mm or 5cm.