To judge from the details in the average computer ad, you'd think these things were built according to a Pentagon procurement manual:

512MB PC2100 DDR SDRAM 533MHZ front-side bus nVidia GeForce2 MX200 graphics card DVD/CD-RW combo drive NIC 2 IEEE 1394 ports QuantiSpeed architecture operates at 2.0 GHz Sound Blaster Audigy.

All that jargon attempts to distract the buyer from a plainer truth: Computers these days are largely commodity products, built from third-party components and stuffed with mostly identical programs. Nearly all of those programs, in turn, have no need for the fastest processor around, or even the 10th-fastest.

As a result, the important things to remember are the same for most home-computing uses.

You'll need 256 megabytes of memory, without which both Mac OS X and Windows XP will grow uncomfortably slow.

The increasing size of programs and data, especially media files, makes a 40-gigabyte hard drive, the minimum available from most companies, equally necessary. Keep in mind that any hard drive will hold roughly 7 percent less data than advertised, thanks to marketing math too boring to relate here.

You'll need a CD-RW drive to copy documents and music to recordable discs (the floppy disk's measly capacity makes it a relic best forgotten, and DVD-ROM drives are of little use on desktop machines).

When it comes to expansion ports, USB 2.0 or FireWire (called "iLink" by Sony and "IEEE-1394" by linguistically challenged PC makers) accommodate data-hungry peripherals such as external hard drives and MP3 players; one or the other is standard on most new machines, as are slower but still essential "regular" USB ports.

Lastly, broadband Internet access and local-area networks, two things likely to be in your PC's future, require an Ethernet port.

Remember those five things -- 256 megs, 40 gigs, CD-RW, USB 2.0 or FireWire, Ethernet. Don't worry about the processor until you consider what you'd do with a new desktop (and don't worry about choosing AMD vs. Intel on any grounds but price).

If you just need a computer to get on the Internet and write letters, then any machine sold today will do. You'll have to make your choice on grounds besides hardware specs; I'll get to those in a moment.

If you plan to buy a digital camera, then budget for more hard-drive space, plus a high-quality color inkjet printer. Easily accessible USB ports will simplify connecting a camera to the computer.

If you're into digital music, a bigger hard drive becomes even more important, and you might as well spring for better speakers, too. With many Windows PCs, unfortunately, you need extra software to save CDs into the standard MP3 format.

If you want to make your own home movies, then you will need a faster processor and as much disk space as you can afford. You will also require a FireWire port to connect most digital camcorders, plus a DVD-recordable drive to save your work. There are two incompatible standards -- DVD-RW and DVD+RW -- but either kind of recordable disc should work in most new DVD players. (For data backup, +RW is better than -RW.)

If you want to play games, especially the fast-paced kind, the most important thing is the graphics acceleration: Do not buy a machine with "integrated" graphics or "shared" video memory; get a separate card with at least 64MB of memory.

Any money left over in your computing budget is best dumped into additional memory -- often cheaper when ordered from a third party -- and then a better monitor. (See the story below for details on flat-panel displays, an increasingly affordable alternative to conventional monitors.)

What else to consider? Among Windows PCs, you have few choices in basic design. Nearly every machine comes in a "tower" configuration that's best parked under a desk but which offers the least convenient access to drives and expansion ports.

Only Gateway and Sony offer smaller all-in-one machines that, like Apple's iMac and eMac, avoid occupying one's entire desk area.

So your choices among PCs may come down to nuances, many of which are easily overlooked in the store. Take noise, for instance. It's possible to build a PC whose fans keep its processor from overheating without generating a whirring, buzzing racket all day long, but some cheaper computers leave out sound-dampening measures.

The humble mouse can be another source of needless irritation. Many desktops today still ship with roller-based rodents that plug into PS/2 ports, instead of optical mice that connect via USB. The latter kind never require you to swab out their innards and can be plugged in and unplugged without rebooting.

Consider expansion potential as well. A PCI slot inside the computer's case will let you add to its capabilities, although most people never bother doing so. More important are those USB 2.0 and FireWire ports on the outside: The more of them, the better, especially if any are on the front of the computer. (PC builders also still include obsolete serial and parallel ports, but maybe they'll come around one of these years.)

Bundled software is usually a disappointment, but if you can get a full office suite (Microsoft or Corel) instead of the usual Microsoft Works, you've come out ahead. The same goes for MP3 and photo-editing software you actually like.

As for technical support, the round-the-clock kind is still best. But that doesn't guarantee that you will reach a human voice when you call, or that the voice will offer useful answers. Keep a tech-savvy friend's phone number handy.

Living with technology, or trying to? E-mail Rob Pegoraro at