Xcode update for 10.6.8
Confirming couple of points


You are running OSX 10.6.8 
Current Xcode version in your machine is 3.2.4


Xcode 4.2 should work fine on your machine. 
I have a snowleopard machine at office running Xcode 4.2, i updated it via the Mac App store only. Ok. If this is not working via Mac App store, then go to your developer account (free or paid) and then download the Xcode version for Snow Leopard and install it manually. Just make sure you are downloading the Snow Leopard edition not the Lion edition. Please don't hesitate to ask for assistance :-) 

Edit : Here i am attaching a snapshot of where to find the link . You need to scroll down and check in the Downloads section.