Apple unleashed a pile of new Mac hardware on its users Tuesday: new 13" Retina MacBook Pros and Mac Minis using Intel's newest Ivy Bridge processors are available for shipping now, while substantially redesigned iMacs begin shipping in November and December. If you like to upgrade when things are new, now is the time to strike.

Any new computer is an investment, though, and you'll want to make sure that what you're buying will last for as long as possible. For each of the new Macs Apple began selling yesterday, the company offers a $200 upgrade option that will turn your Core i5 processor into a Core i7 processor (and while it hasn't started selling the new iMacs yet, those computers should have a similar upgrade path). If you're in the market for a new Mac (or any computer, really), do these CPU upgrades give you a good bang for your upgrade buck?

What's in a number? Well, it depends...

So what's the difference between a Core i5 and a Core i7? Well, thanks to Intel, the answer isn't simple.

The iMacs use desktop-class processors rather than the mobile processors used by the MacBook Pros and Mac Minis, which makes things a bit easier: a Core i7 upgrade will get you more processor cache, more clock speed, and Hyper-threading. There is no Core i5 desktop processor with Hyper-threading and there is only one seldom-seen Core i5 desktop chip with only two cores (the low power i5-3470T), which draws a mostly clear line between the Core i5 and Core i7 product lines—on the desktop, dual-core CPUs are a market segment left to the Core i3, Pentium, and Celeron chips. There are some oddities and variations within these product families, but in general it's easy enough figure out what's what.

It would be all too easy if Intel's mobile lineup followed the same rules as the desktop chips, but they don't: Mobile Core i5 processors are all dual-core parts with Hyper-threading, rather than quad-core parts. Mobile Core i7 processors can be quad-core chips—as they are in the $799 Mac Mini—but they can also be dual-core CPUs with Hyper-threading enabled, making them much less of an upgrade over their mobile Core i5 counterparts. These dual-core i5 and i7 chips also share other marquee features—the most important is probably Turbo Boost, which can greatly increase the speed of processor cores if your computer isn't using all of them at once. Core i3, Pentium, and Celeron processors in both desktops and laptops lack this feature, but all of the i5s and i7s have it.

Since the processors used by the new 13" Retina MacBook Pros are all dual-core chips, when you spend the $200 to upgrade to an i7 CPU, all you're really getting is a little extra clock speed and a little extra cache, which makes much less of a performance difference than the upgraded Mini's two extra cores.

Do you need to spend the cash?

The answer to that question varies depending on what kind of Core i7 processor we're talking about. If you're jumping from two cores to four (as you do in the Mac Mini), you'll probably be happy if you spend the cash. Apple doesn't list Intel's CPU model numbers, but if you're buying from another manufacturer you can usually tell the difference by seeing whether the processor has a Q (for quad-core) in its name (i.e., the Core i7-3610QM). If you're simply getting a slight clock-speed bump and maybe Hyper-threading out of the deal, as is the case in the new iMacs and the 13" Retina MacBook Pro, your money is going to be best spent elsewhere unless you regularly do very CPU-heavy tasks like video editing and transcoding or very heavy Photoshop work, and the minutes and seconds you'll save with a marginally faster CPU are important to you.

For most general-use workloads—browsing the Web, word processing, checking e-mail, infrequent or light Photoshop or Premiere usage, and even gaming—most applications simply don't use more power than is afforded by a modern Ivy Bridge CPU most of the time, especially if we're talking about the quad-core models already shipping with every iMac. Most of the time, that $200 CPU you bought is going to be sitting idle, something that can be done just as capably by slower, cheaper chips.

If you're looking to upgrade either of those Macs (or any other PC), your money is better spent elsewhere: spending $300 will get you a 256GB SSD in the new MacBook Pro, taking the amount of available space from tight-but-workable to just plain workable (depending on the amount of data you need to store locally, of course). In the new iMacs, the 21" models now come with slow-spinning 5400RPM mechanical hard drives, which is actually a downgrade from the 7200RPM desktop hard drives in the 2011 models—there, your upgrade cash would be better spent on a performance-enhancing SSD or Fusion Drive. Things are even better on the PC side, where prices for SSDs have fallen under a dollar per gigabyte and clearance sales can get you nice, fast, high-capacity drives for even less.

It's not that CPU upgrades are totally without merit, or that there aren't people who would benefit from upgrades to either of these new Macs. However, if your upgrade money is limited, CPU upgrades simply don't provide the best bang for your buck.