I suggest you go 4GB of RAM if you can. I can't answer how smooth or fast Mountaln Lion will run in your Macbook.
You can judge that yourself since you're already running Lion. Is it smoother and faster? Mountain Lion might be a little slower with the RAM you have.
Because you already purchased Lion, you might have to pay for Mountain Lion when it comes out.
To be sure, give apple a call and find out.
You will have to pay for ML and you are ineligible for a refund per http://www.apple.com/itunes/affiliates/resources/documentation/us-affiliate-term s-and-conditions.html "all sales are final."
Second the tech specs for ML are here http://www.apple.com/osx/specs/ But Like bimmer mentioned i'd advise at least 4GB of ram, 8 GB if you can manage.
You'll just have to wait it out until Mountain Lion is released then come back and see what kind of experiences folks are having with systems similar to yours. One thing I would do - even running Lion - is to think about upgrading your RAM to 4GB from 2GB as your model, if an early 2011, will support up to 6GB, or 8GB if it's a late 2008.
I think that the RAM increase alone will speed things up and, if your system is up to the ML specs, go for it.