What is the best version to use as a Virtualbox host OS? Is it 12.04, or 9.04? I get the notion that an older system runs faster. Is this true? I especially have to consider the amount of RAM available as well as usage by the host.
You should never use 9.04 as it is old and has security holes. Also, being old, the Virtualbox in the repositories for it has not been updated so it is also full of bugs. 12.04 will do fine to run Virtualbox with a guest.
You can opt to use Unity 2D if you are having low graphics performance on the host, by clicking the circle when you are prompted for your password at login and selecting Unity 2D.
If it is still too slow, you can use 10.04 as it has somewhat less RAM usage while being more up-to-date, or you can install an alternate desktop enviromnent. Lubuntu appears to use the least RAM, and if you are taking this route, there is no need to use 10.04 and you can use 12.04 without concern for performance.
With that said, you will still have issues of you don't have enough RAM(at least 1 GB is recommended, but you can make do with 768MB), no matter what version you use.