Hi Raphael,
There is no way to interact directly with the memory on the iPad and do things like you suggest. The iPad only has a 1 GHz cpu and 256 MB or ram. The memory management part of iOS handles everything.
If you are using iOS 4.2.1, go to the multitasking bar, tap the home button twice, and terminate all apps. Apps fall into three categories: "recents" that are not running at all, suspended or sleeping, and running in the background. The latter are the ones that will affect your available memory and cpu. So if you are concerned about memory, terminate all apps that appear on the multitasking bar.
Other approach is to power cycle, turn off then back on, the iPad. That will stop anything running and free all memory and cpu.
Ralph