64-bit is here for a while. A lot of modern '64-bit' CPUs only support 40-bit physical addresses, so are limited to 'only' 128GB of RAM. Most support 48-bit virtual addresses (the top bit is sign extended, so all 1 or all 0 depending on whether you've got a kernel or userspace address), limiting you to 'only' 32TB of virtual addresses. If RAM sizes continue to double once every year, then it takes another year to use each bit. We currently have some machines with 256GB of RAM, so are using 41 bits. 64 bits will last another 23 years. RAM increases have slowed a bit recently though. 10 years ago, you always wanted as much RAM as possible because you were probably swapping whatever you were doing. Now, most computers are happy with 2GB for programs and the rest for buffer cache. As SSDs get faster, there's less need for caching, but there might be more need for address space as people want to be able to memory map all the files that they access...