There is no need for that intermediate step, if your mac can support MLion you can go straight there, but first check out to see the details at this link reagrding MLion system requirments,
You can go to 10.8 directly from 10.6 if your machine supports it.
However your going to lose your PPC based programs and hardware drivers that are running using Rosetta in 10.6 (not in 10.7 or 10.8)
Also your machine could possible slow down quite a bit, as well as radical UI changes.
Don't say I didn't warn you.
You can clone your present 10.6.8 boot drive using Carbon Copy Cloner to a blank powered external GUID OS X extended journaled formated drive.
If you decide 10.8 is a drag, you can hold the option key down and boot off the 10.6 clone and erase the entire drive and reverse clone.
You also can keep the 10.6 to run programs 10.8 won't.