No. But you probably need to go further than 10.6.3. If you have already bought the retail disc you should read the link below (just to double check).
Minimum specs for Snow Leopard. Mac OS X v10.6 Snow Leopard -Read the Technical Specifications
Once you have bought and upgraded you will need to apply the Combo updater for Snow Leopard. Mac OS X 10.6.8 Update Combo v1.1
And follow that by software update to pick up any iTunes or security updates that have since become available.
As Seventy-One said, normally you will not lose any data. But it's strongly recommended that you have a good backup, at least of any critical data, before you upgrade just in case something goes wrong. I would suggest making a clone to an external drive of your entire hard drive using a utility such as Carbon Copy Cloner or SuperDuper.