Theoretically 64 bit systems can have up to 127+ Billion Gigabytes of RAM before you need a 128 bit systems just like the 32 bit's limitation is 4 Gigabytes.
Scary. But I don't think that will happen. I mean how much more stuff can they add? Pretty soon the whole concept of a PC will be changed and you will like go into a virtual world thingy. Just look at the games and stuff now.