One of the problems with trying to discuss "memory" on the iPad is the fact that Apple (intentionally, I think) confuses "memory" (RAM) with data storage. You won't ever run out of "memory" if iOS is doing its job. On the iPad2 the RAM is 512K. For the most part it doesn't matter how close you come to using that entire amount with running and suspended apps though you may find that killing some apps improves performance. If you don't do that, iOS will chug along swapping apps and suspended apps in and out of RAM as required.
You may, however, run out of data storage if you download enough data (e.g. tunes, apps, books, movies, etc.) If you come close to the storage limit of your iPad, you won't be able to download more data until you remove some "stuff." But the iPad will continue to function.