Well, you could be talking about 2 different things, so I'll try to answer both:
If you're talking save RAM, it's system's internal memory is good enough if you play a few action games here and there. Many RPGS or some games take up a huge chunk, so you might want a Save RAM cart, and unlike the PS1 unit, one cartridge will be mostly likely good enough to save ALL your game collection's worth of memories.
If you talking about the system's RAM available for games, this is a different issue. Most Saturn games will run fine on by itself (in fact, all US released games will), but a few Japanese games require an expansion cartridge mostly Capcom and SNK's fighting games - SFZero3, KOF98, etc...
Here's a link that shows different expansion carts available for the Saturn.http://www.shinforce.com/saturn/information/Cartridges.htm