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Macworld - The world has a lot of unwritten rules -- in social etiquette. In baseball. And in buying computers. For years, we have unquestioningly followed numerous unwritten rules when buying a Mac.

Like many customs, these rules were once based on a foundation of facts and reason. But in the past few years, many long-standing Mac truths have been upended. All Macs run on multiple-core Intel processors now. IMacs are no longer hobbled by crippling feature limitations. And speedy external peripherals have drastically lessened the need for add-on cards.

In other words, the old rules no longer apply. If you're planning on buying a new Mac, you need facts about the modern lineup so you can choose the computer that's right for you.

In this article, we take a look at some common assumptions and explain whether they align with today's realities.

Old rule: I'm a power user; therefore I need a Mac Pro.

For years, Apple Inc.'s high-end Power Mac desktop systems were a great -- and perhaps the only -- choice for a wide variety of Mac users. Many Macworld editors, for example, would never have considered anything less when buying a new Mac.

Power Macs had the fastest processor speeds and internal architectures, not to mention space for lots of RAM, hard drives and expansion cards. Even when the iMac made its debut, it wasn't a computer that serious power users could consider; the first iMacs offered the sort of limited power you'd find in a Mac laptop.

But as the Mac entered the Intel era and the Power Mac became the Mac Pro, something interesting happened. Those lower-end systems became powerful in their own right, down to the dual-core technology that was previously the provenance of the highest-end machines. There's still a gap between high- and lower-end Macs, but now almost every Mac is suitable for general use, even by a wide swath of power users.

In the meantime, Apple has started aiming the Mac Pro at a much narrower and higher-end group of users. Its four- and eight-core processor architectures are ideal for 3-D rendering and scientific applications. But they won't help you build a presentation or write an e-mail message any faster.

Of course, there are speed differences across Apple's product lines. The Mac models that lack dedicated graphics processors -- Mac Minis, MacBooks and MacBook Airs -- offer a fraction of the 3-D performance of other Mac systems, making them unsuitable for fast action games with intensive graphics. And the eight-core 2.8-GHz Mac Pro ran Speedmark almost three times as fast as the slowest Intel Mac, the MacBook Air.

But for most mainstay applications, the high-end iMac and MacBook Pro models are plenty fast (the 3.06-GHz build-to-order iMac even beat the Mac Pro in some of our tests). Even Adobe Photoshop, a heavy-duty program that conventional wisdom has long argued should be run only on a high-end system, works acceptably well on just about any Mac (unless you're editing gigantic files).

And there are a few other reasons to consider an iMac instead of a Mac Pro. The Mac Pro is a large computer that also requires an external display, while the iMac fits into smaller spaces and requires fewer cables than the Mac Pro. And the iMac is much quieter than the Mac Pro, so people who are sensitive to noise will prefer the iMac.

New rule: For many power users who once bought a Power Mac by default, the Mac Pro is overkill, and an iMac or a MacBook Pro is powerful enough. The Mac Pro does exist for a reason -- as an option for people who need the utmost from a Mac, particularly to perform tasks that can take advantage of four or eight processor cores.

If you're performing intense scientific calculations, capturing and editing uncompressed HD video aided by the latest PCIe card, or connecting six displays to a single computer, a Mac Pro is a great choice. But if you're buying a Mac Pro rather than an iMac purely out of pride, we offer this advice: don't.

Old rule: I need a Mac Pro because it's the only expandable Mac, and I need an upgrade path.

For many computer users, expandability is a little like insurance. What if you want to add a hard drive? Or a new video card? Or more RAM? Or a faster processor?

If your computer is truly expandable, you can theoretically stave off obsolescence with a series of canny upgrades over its lifetime. But most people don't really take advantage of their computers' expandability, especially the unique form of expandability the Mac Pro offers.

The Mac Pro is the only Mac with a traditional open-case design, offering easy access to internal hard drives and a panoply of expansion-card slots. But these days, you can attach most peripherals via a Mac's USB, FireWire and Ethernet ports -- including speedy external storage devices and plug-in TV-capture hardware. It's extremely easy to install a new hard drive in a MacBook, and installing RAM in most Macs is also quite simple.