My first thought would be that you could have huge environments with lots of detail. You might be able to pre-load a level as you got close to the end of the current one because the memory was available. Physics calculations might work more quickly when longer numbers were needed because the processor would be handle them in fewer cycles.
Can you imagine a computer that doesn't have a hard drive (that isn't like a trs-80 or apple II, that is)? It would be awesome! 20 gigs of ram would serve as both temporary storage and permanent storage. THe ram could be powered when the computer shuts off and there would be a battery just in case the plug is pulled. 20gigs of ram would cost $5000US at the moment, so it would be possible if we had 64bit processors .