The Mega ST and STE's have built in SCSI hard drives. A Mega2 has 2 MB of RAM and a Mega 4 has 4Mb of RAM. The Mega series has an seperate keyboard. They used the same case as the TT030's.
I use to use my Mega STE4 with games and had no issues running them. On that note I wasn't a huge game player and had less than 30 games.