The factory recoil spring weight is 17lbs. ISMI offers recoil springs up to 24lbs. You can see a few choices here http://bit.ly/iYhdYq
Watch the ejection pattern of your brass to get an idea of how your recoil spring is performing. If the brass is rolling off your knuckles you need less spring. If the brass is flying out there 10+ feet, you need more spring. Ideally, look for the brass to land 2 to 4 feet away.