While the phrase "640KB RAM ought to be enough for anybody" was falsely attributed to Bill Gates, it definitely outlines a way many of us think about the computing limits we're faced with. At the time this was purportedly said, computers were just transitioning to 16-bit architectures with the Intel 8088 processor (the one used in some of the first PCs), which broke through the 64-byte memory limit of the older architecture.
In the past few years Macs and other PCs have been in a similar transition with 64-bit capable processors (starting with Apple's G5), but while the hardware has been capable of 64-bit for a while, and even though Apple pushed "64-bit" as best it could, the software implementations have lagged behind.
While prior versions of the OS had some degree of 64-bit support (mainly for memory addressing to allow for more than 4GB RAM to be recognized by the system), many processes including the kernel were still only 32-bit. It was not until OS X 10.6 (6 years after 64-bit hardware became available) that we saw the first fully 64-bit version of the Mac OS; however, Apple initially made a 32-bit kernel be the default for the client version of the OS.
This confused a number of people who, fully expecting the "64-bit" experience, felt undermined and found ways to boot into 64-bit mode. These included holding hotkey combinations at startup, as well as editing system files and using utilities to make 64-bit mode the default. People who did this were eager to see major capability boosts to their systems, but instead realized third-party software was not yet developed enough to work in the new environment.
One primary limitation is system extensions which, like application plugins, need to have their bitness match the software they are enhancing. Therefore, many people realized that some applications and extensions would not work when booting to 64-bit mode, and ended up resorting back to the 32-bit kernel.
Because of these limitations Apple kept the 32-bit kernel, but additionally provided options to easily change the bitness of user applications (with the "get info" menu) so older plug-ins would still work with those applications.
This has been the situation for OS X for some time, but currently most applications, plugins, and extensions have a 64-bit version available so there is no point to maintaining a default 32-bit kernel, especially for the systems used by creative professionals. Even small tools such as Apple's TextEdit have a 64-bit versions available, and while most will still not need more than 4GB of RAM despite being capable of it, sooner or later this amount will be used by applications. As a result, in the most recent 2010 Mac Pro systems Apple is changing the default kernel to be 64-bit, and perhaps we will see this be the default for other systems in the future as well.
Now instead of holding the "6" and "4" keys at startup to get to 64-bit mode, on the new Mac Pro systems you will have to hold the "3" and "2" keys to get to 32-bit mode, if needed.
These days computers are loaded with graphics and visual effects, as well as advanced capabilities for performing varieties of tasks, many of which store and process huge amounts of data. There's no question the amount of RAM required in a system is surpassing the previous 4GB maximum and making better use of the 64-bit architectures we all have. With the transition to 64-bit and with more systems now fully capable of addressing more memory, instead of 640KB as the huge potential for RAM usage, we're at over 17 exabytes (17 billion gigabytes) of potentially addressable RAM. Now THAT ought to be enough for anybody, or is it?