There are two different types of memory on your phone. RAM (short term memory) and ROM (storage memory). You will have problems if either of them get too low. If you are running out of space for email, text, apps, etc then it is your ROM that is used up. The ROM is much like a hard drive on a computer and it won't allow you to fill it all up.

You say that you have Apps2Sd, but do you know if it is working right? If it is, then you shouldn't have any apps on your phone and they should all be on your SD card.

I would look at how many text messages you have on the phone. If you have any threads with more than 100 or 200 messages in it then it would probably be a good idea to start deleting them. Those can take up a lot of space.

You could try clearing cache. Go to Settings > Applications > Manage Applications (wait for everything to show up with size and icons) then press the menu button and sort by size. This should give you a listing of all your apps with the largest at the top. If you open each app from here you will the amount of space they are taking up on the phone and how much cache they have stored. There should also be a button that will let you clear the cache. Doing this will free up a lot of memory for you.

Also, how many apps do you have installed? Are you using only gmail or do you have another mail account going to the phone? What OS are you running?

 

Well, if you are able to choose which apps you want on the SD card I would guess it is 2.2, but to be sure go to Settings > About Phone and look at Firmware Version.

Also, do you have the Droid, Droid 2 or the Droid X?