Two things:
Yes, you can put apps on the SD but you have to root. The ROM I user is xtrSense and includes Apps2SD.
Second, I've had this problem with low memory. Rooting and installing xtrSense well free up a LOT of memory. But if you don't want to root, go to settings, applications and manage applications (or something like that), then sort by size. My bet is Gmail and/or your web browser are holding a lot of data. You can clear the cache in your browser and clear the data in your Gmail (it will not delete your Emails, since those are synced, it simply clears data on your phone).
I highly recommend rooting, but if you don't want to then try checking your app cache and data.
Sent from my rooted Droid Eris 2.1 (xtrSense)