Many people anticipate huge performance gains from the newer CPUs that can process 64 bits of data at a time, compared to the 32-bit CPUs that were in use for years. Sounds twice as fast, right? Well, not automatically. To get the most out of a 64-bit CPU, you also need to have the following:
A 64-bit Operating System. For example, it isn't enough to have Windows 7; you need the 64-bit version of Windows 7. (Photoshop CS5 doesn't officially support 64-bit Windows XP.) In Mac OS X, you'll want to use Mac OS X 10.6 for the best support of 64-bit capabilities.
Well Over 4 GB of RAM. One of the biggest benefits of 64-bit computing is that Photoshop can directly use more than the roughly 3 GB of RAM that it can use under 32-bit computing. If this appeals to you, don't upgrade to just 4 GB or you won't see much difference. Aim for 8 GB to start, and go higher if your files are big enough to need it. In 64-bit Windows 7, the edition you use determines the maximum amount of RAM the system recognizes, ranging from the 8 GB supported by the Home Basic edition to the 192 GB limit of the Ultimate edition.
Really Big Files. The ability of a 64-bit processor to directly address much more RAM speeds up the processing of very large files. If you work with Photoshop files that are over 1 GB in size, you should see major performance gains from 64-bit Photoshop. But if you mainly make simple edits to 5-megapixel JPEG camera files without using many layers, masks, or Smart Objects, 64-bit Photoshop probably won't feel much faster.
Choosing 64-bit over 32-bit computing is like driving a 64-passenger bus instead of a 32-passenger bus. The 64-passenger bus can potentially move twice as many people in a single trip. But if you rarely carry that many people, the 64-passenger bus is no faster than the 32-passenger bus, and because it's bigger, it may actually cost you a bit more in overhead when it isn't being used to capacity. A 64-bit system can potentially be slower than 32-bit when editing small files or when not much RAM is installed.