I'm running 10.6.3 (Leopard)
First, you need to clarify what you're currently using, because 10.6.3 is not Leopard, it's Snow Leopard.
Snow Leopard is only available on a DVD, it can't be downloaded.
If you actually have Snow Leopard 10.6.3, then you can update to 10.6.8 by using Software Update.
Check the minimum requirements for ML here: http://www.apple.com/osx/specs/
You could also serach the forums, "More Like This," as this question is asked and answered almost every day.