More bits gets you access to more memory. The processor inside your PC communicates with your system memory (RAM) with numeric addressing. Thus the maximum amount of memory a 32-bit processor can address is 232 bytes, or 4 gigabytes. Newer 64-bit processors—not to mention the 64-bit operating systems that run on them—can address 264 bytes of memory, or 17,179,869,184 gigabytes (16 exabytes) of RAM. (17 million gigabytes may sound like a lot of space now, but it won’t be long before you’ll be taking baby pictures with an 8-gigapixel digital camera.)
Windows NT, released in 1993, was Microsoft’s first fully 32-bit operating system. But it took eight years before the platform, which had since evolved into Windows 2000 and then XP, became mainstream. (For those keeping track, Windows 9x doesn’t count because it was a hybrid OS that ran 32-bit applications on a 16-bit DOS foundation, which was one of the reasons it was so unstable.) 64-bit Windows became a reality in XP, but Vista is Microsoft’s first serious attempt to make 64-bit computing mainstream. But the question is, how mainstream is it?