Personal computer enhancements make great Christmas gifts. But unless the computer buff on your list has told you exactly what to get, it's hard to know. Some suggestions: Nearly everyone has a word processor, and most of them have a spelling checker built in. But not all have a thesaurus, and there's none better than Word Finder from Microlytics Inc. (300 Main St., East Rochester, N.Y., 14445). The program is available for the IBM PC and compatibles and the Apple Macintosh.

Word Finder loads into your computer's memory, where it remains in the background as you use your word processor. When you invoke it by hitting the designated "hot key," it checks the word at the cursor, then searches its word list for synonyms. In seconds, possible synonyms appear in a window on your screen. Choose the one you want by moving the cursor to it, then hit the enter key. The new word instantly replaces the old one in your text.

The program works best if you store its 220,000-word dictionary on a hard disk. But it also works with floppy disks, and there is a shorter, 90,000-word list for those with limited disk space. It lists for $79.95 for the PC and $59.95 for the Macintosh; mail-order houses and software retailers sell it for less.

Another tool for writers is RightWriter, from RightSoft (2033 Wood St., Suite 218, Sarasota, Fla., 34237). This program evaluates writing for style, grammar, punctuation and usage. It is available only for the IBM PC and compatibles.

It is simple to use. The program examines text and grades it for readability, strength, descriptive quality and use of jargon. It also creates a second, marked-up version of your file, which you can print or read onscreen.

The marked-up copy points out unexplained adjectives and long or complex sentences, suggests replacing big words with simpler ones. It also notes the use of the passive voice.

Not all of RightWriter's comments are valid, but enough are so that this program can help anyone write more clearly, simply and forcefully. It sells for $95, which is steep, so look for a mail-order discount.

A cheaper alternative, also for the IBM PC only, is PC-Stylist from Buttonware (P.O. Box 5786, Bellevue, Wash., 98006).It works much the same way as RightWriter, generating overall grades on readability, "personal tone" and "action." It also reports on such aspects as length, number of long words and words per sentence. Price: $29.95 from Buttonware, which includes telephone support, if needed.

PC-Stylist is "shareware," which means it can also be obtained free from a computer bulletin board or at a nominal charge from a user-group library.

Nearly anyone with any kind of computer can use a printer buffer. Here's how one works: Tell your computer to print something and instead of it making you wait for the document to be done before using the computer again, the data flows into the buffer. It acts as a waiting room for the printer while you go on working.

There are software printer buffers (usually called "spoolers"), but hardware buffers are faster and better. They also are expensive, except for one: the MicroStuffer from Supra Corp (1133 Commercial Way, Albany, Ore., 97321).You can get this 64K buffer for $69 from Central Computer Products (330 Central Ave., Fillmore, Calif., 93015, phone 800-533-8049).

An even cheaper gift idea is "Thingi," from Advanced Gravis Computer Technology (6894 Palm Ave., Burnaby, B.C. V5J4M3, Canada). It is a thin strip of plastic, like a ruler, twisted at one end so that it can attach with velcro to the top of your computer's monitor. The "Thingi" sticks out to either side and, with a plastic clasp, acts as a paper holder. It works well and costs about $10.

Finally, something every computer user should have: a modem. It opens a world of possibilities, from communicating with others via electronic mail to downloading free software from bulletin boards.

Prices of external modems have plummeted in the past year. Here are two outstanding bargains: The Kyocera 1200S, a well-made Hayes-compatible, 1200-baud modem that has been discontinued. It is available from COMB Authorized Liquidators of Minneapolis, 800-328-0609, for $79.

Also, the Practical Peripherals 2400SA, a 2400-baud modem. It lists for $239, but some dealers are selling it for less than $200. Try the Egghead Software chain, which gives a 14-day money-back guarantee, or PC Connection, a mail-order house in Marlow, N.H., 800-243-8088.

Brit Hume is a contributor to the Washington Post Writers Group. He is an ABC News Capitol Hill correspondent and founding editor of a computer newsletter.