It's hard to say exactly what your problem is since your setup is obviously unique, but the top three things I can think of are to remove wallpapers and ringtones, clear browser cache and delete old messages/make sure attachments are saved to storage card. (Start > Messaging > Menu > Options > Storage tab > check the box). Also I'd recommend using the new
Opera Mini which is much faster/better/easier to use than anything out there, plus it uses less data and has no cache since all of the page loading is done on Opera's end - meaning battery life should be better, and at the very least, your memory won't fill up at all.