As Nick stated, try and get one of the same value. If one of the same value is not available, you can always go up in CCA/ah, but never go down. Vehicles are spec'ed with a certain battery because that is what it needs to operate the starter to get the engine going. With a lower CCA/ah, you run the risk of dragging the starter, which can cause it damage. The specific CCA/ah also gives the accessories enough power without over taxing the alternator to keep up with the demand. It's kind of a symbiotic relationship.
The biggest thing you'll need to worry about is form factor. If the battery has too large of footprint it won't fit in the hole and allow you to bolt it down. If it's too tall, you won't be able to shut the hood, and shorting top post batteries across the hood is not a good thing ... think of thermal nuclear meltdown on an automobile scale.