How did you get Minecraft running with Java 7? Did you simply install it from the repository?
I only ask because of the problems I had getting Minecraft to run smoothly on my desktop (also with a Radeon HD 5770) under Debian Wheezy. Essentially, Ubuntu has inherited Debian's system for dealing with multiple version of packages, for example: Java. So if you 'sudo aptitude install openjdk-7-jre' from the repository, Ubuntu will still use OpenJDK 6 as its JRE because that is the default version.
Fortunately, you can rectify this problem fairly easily. First, run the following command to list all versions of Java installed on your system:
$ update-java-alternatives -l
java-1.6.0-openjdk-amd64 1061 /usr/lib/jvm/java-1.6.0-openjdk-amd64
java-1.7.0-openjdk-amd64 1051 /usr/lib/jvm/java-1.7.0-openjdk-amd64
Next, copy the name of the Java installation you want to set as default (from my example output it would be java-1.7.0-openjdk-amd64
) and run the following command:
$ sudo update-java-alternatives -s java-1.7.0-openjdk-amd64
You can safely ignore the 'error: no alternatives for something-or-other.' messages that it outputs. That's normal. It just indicates that you don't have all the optional Java components/libraries installed from the repository.
That said, while I certainly experience much smoother game play with OpenJDK 7, I don't think that's actually your problem. While I'm not sure of the exact performance penalty that Unity inflicts on Minecraft, any compositing window manager will cause a drop in frame rate while in game (Unity 3D -- the default desktop environment in Ubuntu 12.04 and the only version of Unity in Ubuntu 12.10 -- uses Compiz as its window manager). If my theory from your previous thread was correct, you should get much better performance with another non-compositing window manager, such as openbox.
I experienced this same problem running Minecraft on my desktop under GNOME 3.4. However, any time I switched to openbox my frame rate in Minecraft went up dramatically. Since I really like GNOME 3 and don't want to log out then log back in using a different window manager each time I want to play a game, my solution is to start a second X session using the openbox window manager for playing games. (You can read a general overview of this method here
.) That way, I can just press Ctrl+Alt+F8 to temporarily suspend my 'regular' session and switch to my 'gaming' session. To switch back I just need to press Ctrl+Alt+F7. Its simple and effective.