PC enthusiasts can often sound like brand-loyal trolls, especially if you get them started on CPUs and graphics. They do occasionally have a valid point, however. When it comes to computer hardware, brands do matter, but why they matter usually depends on the hardware in question. Here are some things you'll want to consider...
Processors: Intel vs AMD
There was a time where Intel and AMD were more fierce competitors, but things are more simple today: If you want the best performance, go Intel. Right now, most outperform their AMD brethren. If you want something that's less expensive, go with AMD.
That would explain why so many people recommend Intel for higher-powered machines, but AMD still has a place in today's market. Because of their low price, AMD processors are great for budget PCs, especially those that aren't doing anything CPU intensive. Furthermore, their current APUs have some awesome integrated graphics processing baked right into the chip, which is perfect for home theatre PCs (like this tasty $700 build) or some light gaming.
So when you ask around forums for help, be sure to mention what you're using the computer for and what your budget is. If you're looking to build a $2000 gaming computer, that could explain why people are recommending Intel over AMD -- you're going to get a better processor for that rig.
Video Cards: NVIDIA vs AMD
Video cards are a little different. When it comes to choosing NVIDIA or AMD (formerly ATI), brand doesn't matter as much as the individual card's performance. At any given price point, NVIDIA might have a better performing card than AMD, or vice versa. For example, if you're looking for a card around $250, you'll find that NVIDIA's GeForce GTX 760 performs slightly better than AMD's Radeon HD 7950. However, AMD might have a card that performs better at other price points.
Of course, performance isn't the only consideration. AMD often bundles free games with its video cards, which -- if you were planning on buying those games anyway -- effectively makes that card cheaper, giving you more bang for your buck. AMD is also preferable when it comes to multi-monitor gaming, while NVIDIA has features like PhysX on some games, or the new GPU Boost feature that overclocks your card as you use it.
Some PC games are tailored for a specific graphics manufacturer, which can lead to performance issues if you have the wrong brand of card. In other words, it pays to check up on the games you're interested in before you decide on which graphics card to buy.
As with processors, we recommend asking around for help, and give as many specifics about your build as you possibly can. The differences are mostly minor, but they are worth looking into when you're searching for a card.
Everything Else: When Manufacturer Matters
You've probably noticed that video cards have two "brands": the chipset manufacturer (NVIDIA and AMD), and the actual card manufacturer (MSI, EVGA, ASUS and so on). We discussed the former above, but once you've figured out the graphics chip you want, you need to decide which model card to buy. If you're buying a motherboard, RAM or other hardware, you'll have to make the same consideration.
Generally, this is where things become a little more complicated, and you have to do some research. With video cards, the main difference will be in what kind of cooling comes on the card, as well as other proprietary features. With motherboards, you'll often find different features in the BIOS. With RAM, it's mostly about looks and reliability. With all hardware, you'll also want to consider what kind of warranty a given brand provides -- even if all hardware features were created equal, a good warranty can make your break your experience if something goes wrong.
We could go into so much more detail than this, but we'd probably end up writing a book on it. The main thing you should take away is that brand does matter, but it isn't super cut-and-dry. Your needs will determine what you buy, and you should ask around to find out what will be the best for you and your rig.
Are you a staunch AMD loyalist or a smug Nvidia supporter? Share your brand preferences in the comments section below and explain the reasons why.