Any questions about my dell d500 batteries

With Lithium Ion batteries on notebook, there's not much of a problem of battery memory anymore, so one can charge a laptop battery whenever, without having to wait for it to completely discharge.

But what's better overall for my battery's life and the environment,if I keep my laptop plugged in at all times when I don't need to go portable, or only keep it in enough to charge it, and then let it run off the battery until it runs out of juice?

If you don't take the laptop anywhere really i would remove the battery and store it somewhere safe until needed. If you do take it places off and on then you can keep the battery in and plugged in most of the time. If your battery life starts to die then charge it fully and let it die completely 3 times..That often resets some capacitors and helps restore batter life.