Which operating system is best? The Mac operating system is superb and works seamlessly with any Apple computer. But bear in mind that the Mac mini is Apple's only budget desktop, and it comes sans keyboard, mouse and monitor. Apple's upgrades and peripherals can get pricey. If you're comfortable with Windows, you'll find a variety of budget-friendly desktop options that run either Windows 7 or the newer Windows 8 operating set up. Keep in mind that a monitor that uses Windows 8's touch features can get pricy. Among sub-$850 options, we didn't spot any standout all-in-one desktop computers with touch screens.
Do you want a traditional desktop computer or an all-in-one? An all-in-one is a modular desktop unit with an integrated display. They look terrific -- taking up less space and producing less cord and other clutter than traditional desktop setups. But all-in-one computers can be pricier than a conventional desktop or sacrifice processing power for display quality. Also, you can't upgrade the monitor and can rarely upgrade components.
What extras will you need? You'll need a monitor, keyboard, mouse or trackpad, as well as cables for these unless they are wireless. Some computer systems ship with all or some of these items, but with others you are on your own. If speakers aren't provided, you'll have to supply those as well. Not all desktops come equipped with built-in optical drives but you can connect an external optical drive or additional hard drive storage via the USB ports. Ditto with a memory card reader.
Will you be using your computer as a home theater PC (HTPC)? If so, you'll want a desktop computer with an HDMI output for sending HD content to your TV (or monitor). As a bare minimum, you can use an HDCP-compliant DVI output and a DVI to HDMI connector. An integrated optical drive is also a plus -- preferably a Blu-ray drive -- but you can also add an external drive post-purchase. Some desktop computers designed for HTPC use ship with a remote control as well.
Is gaming practical on a cheap desktop computer? Sure it is, but you need to keep your expectations in line. Most cheaper desktop computers rely on integrated graphics. Some have discrete graphics, but use a low-end graphics card. In light of that, the most advanced 3D games won't play at acceptable resolutions or frame rates on a sub-$850 system.
Some systems are upgradable at purchase -- or later on by the owner -- to use more advanced graphics solutions, but that adds to the ultimate bottom line and moves such desktop computers out of the budget class. However, all but the lowest-powered systems can handle less intense games without a hitch. If there's a specific game you know you'll want to play, check its hardware requirements and choose appropriately.
Do you want the flexibility to upgrade your system as your needs grow? All-in-ones and compact desktops typically offer no or very limited expandability. If you'd like to add more memory, additional hard drives, graphics cards, etc. later on, your best bet is a traditional full-sized tower.