Technically, nothing..
That can run a lot of the 1 and 2 floppy OCS games...
I did that for a bit...
All you really need to be able to run the rest of them is more RAM.
You can do that with a PCMCIA RAM card, although that actually slows down the 1200 a bit. I know it works tho, as I did that for a bit before I got my internal RAM, and I didn't notice the slowdown at the time. (I only put the PCMCIA RAM card in to play games, I didn't leave it in for general stuff.)
However, better is an internal RAM card. 4M is plenty (in fact, more than 4M can cause problems with PCMCIA cards if you plan on doing that).
EVEN better is an accelerator with RAM. ;-)
desiv