I get this one a lot, and it's hard to address because it's a moving target. Computer hardware updates so rapidly that anything I post here is going to be horribly dated in six months. So, I suppose the answer is to make this an ongoing series and update it every so often. With the new Macbook update, this is a good time to start.
First, PC vs. Mac: buy whichever one you like better. Seriously the two operating systems have never been more similar, and a Mac can run PC apps easily via Parallels or Fusion--and there aren't any killer Mac apps that your PC is missing. Yes, there's Linux, too, and we'll get to that later*.
If you don't have a preference, buy what you're used to, which will probably be a PC. If you've never used a computer before, ask the person who's reading this to you what they prefer, and go with that; there are no serious operating system features in either that are worth more than working with something familiar, and what your nearest technically-oriented person knows.
If your needs are very light, you can get away with a "netbook", which is a small, inexpensive laptop without a lot of power. All you can really do with these is email and web surfing, and maybe light word processing, but the tiny keyboards can make that difficult. These will run between a few hundred and a thousand dollars, and only really exist for PCs; the Macbook Air is the closest that Macs get, and it's really a lightweight, fully-functional laptop, and has a cost commensurate with that.
If you're looking for a Mac, this will be easy, because there are only a handful of models to choose from. The Mac Mini is the entry-level Mac desktop that works with your existing keyboard/mouse/monitor, and is astonishingly small. The iMac is their more standard desktop all-in-one (and the only exception to my later advice about all-in-one machines). The Macbook line is for notebooks, and most will be happy with the Macbook Air, the less expensive and lighter version; the Macbook Pro is for those with greater needs and greater cash. The Mac Pro is the full-size desktop and only really necessary for professionals who have need of really impressive hardware to work on.
For a PC, we'll need to choose a make first, and I can only really recommend either Dell or Lenovo these days. Sony makes some nicely designed hardware, but they install a bunch of extra software on top, and the cost is unjustified. HP used to be a nice place to find hardware, but if you haven't heard the stories they've gone badly downhill in the last few years. Look at the two linked above, and compare prices, and you'll end up with something respectable.
Now for the really perishable information: for a Mac or a PC (laptop or desktop), you'll want at least 4GB of RAM for just basic computer use, 8GB for basic video gaming and more serious usage. 12GB-16GB if you want something that really screams. RAM (random access memory) governs how large a program you can have running, and how many can run at once. The operating system itself (whichever it is) will use around 1-2GB; whatever is left after that is available to programs.
If you're buying a Windows PC, make sure you get 64-bit Windows 7.
Macs and Intel-based PCs should have a Core i3 or better--ideally an i5 or (for something really powerful) an i7. AMD-based PCs should have something preceded by A4, A6, or A8.
A "hard disk drive" (HDD) will have a lot of space, but a "solid state disk" or SSD is going to perform much better. If you can live with the more limited storage, the latter is very much recommended, but check your storage needs first, and see how much your current machine is using.
Some things to avoid:
- Acer or Toshiba hardware. These manufacturers use bottom-shelf parts and have the worst warranty support in the business. They're always about 10% cheaper than the equivalent elsewhere, but you should buy an HP before either of these.
- All-in-one machines, unless they're iMacs. These marry the most perishable piece of equipment in your setup--the computer itself--with the one that best retains its value, the monitor. It means that, when it comes time to upgrade, you'll be throwing out the screen with the rest of the computer. iMacs are the exception, not because this doesn't apply, but because Macs on the whole retain their value better and use more reliable hardware.
- Building your own PC, because while you used to be able to save money doing this, that's no longer the case. Manufacturers move so much volume these days that they get enormous economy-of-scale discounts, and the labor component of building a PC is minuscule--for someone who builds PCs all day. You can expect a couple of hours of putting things together and installing your operating system, and even then you will have spent more on parts than buying something prebuilt.
* Linux, if you don't know, is a free operating system, one that comes in many different versions. The most popular are variants of Debian Linux, in particular Ubuntu. You can save some money by going this route, but there are some big caveats. You won't be able to (easily, before the Linux folks start shouting about Wine) use your Windows programs, so you'll be stuck learning new software. Linux is more rare and thus harder to find someone to support. And Linux is largely locked out of Netflix, which is probably the single biggest barrier to entry out there. If and only if none of that worries you, a Linux machine can be a great choice. You can usually get away with older, less-powerful hardware, which can save you some money as well. Both Lenovo and Dell have stopped selling pre-built new Linux machines, but the Dell Outlet sells older, refurbished machines that would suit Linux well, and installation has never been easier; you can download Ubuntu directly or order CDs and DVDs.