Your PC

Windows vs. Mac vs. Linux - Windows vs. Mac has long been a perennial debate, and it's still a personal decision as to whether that OS is right for you. But now desktop Linux is on the rise, complicating things even further. It's all very confusing, but here's some advice: Don't jump from Windows to a Mac or Linux without spending a little hands-on time with the OS, either at a physical store or a friend's house. Both are very similar to Windows in many ways, but some substantial differences remain. I regularly recommend both alternatives for readers, but not unless they've experienced Mac OS or Linux in the flesh first.

Desktop vs. Laptop - Most people know this answer coming in, but many are still confused about whether they should go portable. A key issue is price: Expect to pay an extra $500 for a comparably equipped laptop vs. a similar desktop (sans monitor). Is that premium worth it to you for the extra mobility? If so, make the jump to laptop. Don't forget, though: Your laptop will be dead after anywhere from one to three years of use, depending on how rough you are with it. A good desktop PC will last you five years or more, and even longer with appropriate upgrades.

CPU - I'm assuming we're talking a Windows Vista or XP PC from here on out, as that represents the vast majority of computer buyers. (Linux and Mac PCs have far fewer choices when it comes to specs, so just roll with what's available.) As for CPU, right now Intel Core 2 Duo is the way to go, especially on laptops. The AMD Athlon 64 or Phenom are still solid choices for desktops, especially if you're on a budget. Don't get Celeron- or Sempron-based systems if you can help it. Also, it's not worth buying the very fastest CPU on the market. A good rule of thumb is to get a CPU that is two rungs down from the top, speedwise. You'll be getting great performance at a very good price.

RAM - This one's easy. In the Vista world, you need 2GB of RAM. Less will slow down your computer. More will do you no additional good. Don't worry about the speed of the RAM, cache, front side bus, or any of that stuff.
Hard Drive - Even an entry-level drive is more than enough for most people, unless you do loads of video editing on your computer. Even starter computers usually come with 250GB of hard drive space or more now. Upgrade as you need it.

Optical Drive - Unless you are set on high-definition DVD, a dual-layer DVD writer (standard on most machines now) is all you need.

Graphics - Unless you're spending under about $1,000 (laptops) or $600 (desktops), avoid integrated or "shared" graphics. They will noticeably slow your system under Vista and any gaming will be impossible. You don't need to break the bank to get a good graphics card; an Nvidia GeForce 8500GT supports DirectX 10 and can be found for a mere $70, for example. PC makers tend to offer only a couple of video card options with new computers, so get what you can afford, Nvidia or ATI, as long as it's DirectX compatible.

Laptop Screen Size - 15.4-inch laptops are the mainstream now. You'll find the best deals on machines at this size. However, plenty of smaller options abound, at 14 inches, 13.3 inches, and even smaller, but I personally find the lack of screen real estate makes me less productive below 15.4 inches. Again, it's up to you... and remember that those sexy ultraportables have stripped-down components (to keep them light) and can cost much more than larger laptops. 17-inch laptops (aka "desktop replacements") are another option, but they are not terribly feasible if you travel with them.