A 64-bit kernel can access more RAM without using Physical Address Extension (PAE), and allow processes to use more than 4GB of actual memory without having to map some of it into virtual memory. So no, it does not actually increase the maximum amount of RAM you can use in a MacBook Pro. A Mac Pro or an Xserve it does to a point (32GB. Greater than 32GB and you'll want to use the 64-bit kernel)
If your MacBook Pro physically supported more than 4GB of RAM both Leopard and Snow Leopard would be able to see it. According to MacTracker your motherboard actually supports up to 6GB of RAM instead of the 4GB maximum Apple says it does.