Faxing is all the rage. And no wonder. You can send a four-page fax for about a minute's long distance phone charge. Night or day, it's often cheaper than buying paper, an envelope and a 29-cent stamp.

If you own an IBM PC or Macintosh computer, think twice before you buy a separate fax machine. Your computer and printer already supply its most expensive parts. For far less than the cost of a bargain basement machine (as little as $200), you can buy most of the missing parts.These beefed-up computers do a magnificent job of receiving and printing faxes. They're convenient and economical. They can store each fax on disk (using about 1M per fax) and print as many copies as you want.

You can fax all kinds of computer-made files, whether word processed, spreadsheeted or drawn. With some trial and error, you can send cover pages that match the pages made by expensive fax machines.

But unlike stand-alone fax machines, you can't fax the words or images right from a page of paper unless you also buy a scanner. Full-page scanners cost upward of $1,000, hand scanners much less but scan only portions of pages at a pass.

We tested fax add-ons from four old-line suppliers: Apple, Intel, Hayes and The Complete PC. Most double as computer-to-computer modems. If you don't already own a 2400-baud modem, the combination's a good buy.

Apple recently dropped its Apple-Fax, so that market belongs to quality products by Orchid. Their modem box plugs in externally. (For dealer info, call (415) 683-0300.) The others insert into a slot inside your computer. A recent column explained how to add the boards. They all come with controlling programs.

All the programs receive faxes in the background. You can be typing manuscripts or analyzing spread-sheets while the program picks up the phone, receives incoming faxes and stores them on your computer disk. If your Mac program only works in the foreground, substitute BackFAX software from Solutions International. (Phone (802) 658-5506 for the name of your nearest dealer.) All the fax boards we tested are mature and worry-free. It's the software they package that now makes the big difference between brands.

Take Intel. Their first fax board looked like a Rube Goldberg invention. Parts were strewn all over. The software was equally awkward. We couldn't send a good-looking fax, only unformatted ASCII files. Lost were italics, underlines and different kinds of type.

Intel's SatisFAXion board now looks sleek and performs smoothly. Moreover, its latest software solves the formatting problem. You needn't send ASCII files to the phone line. It intercepts, interprets and reroutes files that are normally sent, formatted, to your printer.

Hayes and Complete PC software do the same thing: Their programs fax files as if printing to a printer. The make-believe printer is only a simple old nine-pin Epson dot matrix model, such as the MX-80 or RX-80. The output looks definitely compu-tery. We find it a small price to pay for being able to zap something right from our computer into a distant fax machine.

On the Mac, the BackFAX software operates in much the same way. There's one big difference. It can handle files set up to print to Postscript printers. You can zap tall and tiny type, grey background tones and dozens of typesetting fonts. What you send looks as good as if you'd used a great stand-alone fax machine.

Our test results suggest some recommendations. For Mac or Apple owners: Orchid fax modem which is bundled with BackFAX. For IBM compatible owners, it depends on your needs.

If price is most important, Hayes JR4800 is cheapest. It comes without a modem and retails for $200.

If you're on a LAN, or expecting to network soon, get Intel's Satis-FAXion. With Conetic Systems' Higgins To:FAX software, a single Intel fax board can accommodate everyone connected in the LAN.

We like the way Intel's software stores and retrieves information. It lets us group names and addresses into subdirectories within one fax-phone database. Other fax software makes us keep separate directories.