Were you waiting to buy a Macintosh or genuine made-by-IBM personal computer until prices fell below a thousand bucks? Apple's and IBM's pre-holiday ads sure make it seem like the time is now.

Before you rush out, figure the real cost. You may be in for some surprises.Take IBM's PS/1 - a thousand bucks list with two floppy drives, 512K RAM expandable to 1 meg, optional 30M hard disk, sharp VGA monitor and built-in modem board. It looks like a huge bargain. But let's peel back the cover.

To start with, it's a brand new model, untested by cash customers. It's either IBM's first miracle computer, or - like all IBM models - early buyers will find a few bugs.

Second, if you buy it you're buying an outdated central processor chip. Intel, the i286's manufacturer, took billboard ads well over a year ago to pronounce the i286 obsolete. The chip works well, as old AT users can tell you. Lots of last year's software runs well on it. But many of the hot new programs and program updates coming in 1991 will require more power.

The only business use we can see for the PS/1 is for adding a computing station to an existing local area network (LAN). But for that, you can get a better deal with other equipment.

Biggest drawback for businesses: There's absolutely no room inside for circuit cards. This is the first full-size computer we've seen come like that in half a dozen years. The lack translates to no room inside for fax, CD-ROM, telephone answerer or any of the other useful expansion boards now available or coming along.

So it looks like IBM, whose middle name is "Business," built the PS/1 specifically to enter the bargain basement non-business market.

How does it stack up as a home computer? It's only half finished. If you like arcade games, they're most fun using a joystick. The PS/1 has no place to plug one in.

You can get around the problem with IBM's plug-in box that connects the PS/1 to accessories like joysticks. It costs $169.

Another $249 buys a circuit card that accepts two joysticks. One comes with the card. For two-player games, IBM charges $39 for a second stick. You can probably substitute Radio Shack's joystick and save 10 bucks.

Here's another must: Don't buy the PS/1 without an extra 128K RAM. The 512K RAM it comes with isn't enough to run today's best games. It won't even run late versions of school-worthy word processing programs such as Professional Write. Add at least $199 to get the 640K challenging games and educational programs demand. Expanding to the limit, 1M, isn't a bad idea. (That limit will be too low for many 1992 programs.)

By now, our PS/1 costs $1,655 or more. But hold on. It has a monochrome monitor. Anyone who's tried to play an intricate game on a one-color monitor knows true frustration. Half the appeal in today's educational programs, too, is due to clever colors. Adding color at the IBM shop brings cost up to - sit down, folks - $2,105.

While you're checking out cheap joysticks at Radio Shack, take a look at the $1,500 Tandy 1000 TL/2. It uses the same i286 chip as the PS/1. While Tandy runs it 20 percent slower, you won't notice that with most education and game programs.

Here's a worse negative: The TL/2 uses a CGA color circuit.

That means fewer colors and fuzzier pictures. Figure on spending $400 to match the color-screen on our upgraded PS/1. It still brings the TL/2 in for $200 less than the PS/1. And since Tandy (unlike IBM) is forever running specials, you're likely to get an even better deal.

Tandy throws in a bonus: a fine digital sound system.

IBM's sound is nothing to brag about. Some of the best early childhood programs, such as MathTalk, make excellent use of sound but good sound is vital to understanding the words.

Whichever computer you buy, we urge adding at least a 20M hard disk, especially if you plan to do word processing. Even games are a pain to play when you're forever having to swap floppy disks. That's another $400 or so on the price tag. Your real cost: $2,500 for the PS/1, $2,300 for the Tandy - minus whatever you save by shopping around and bargaining hard.

So much for IBM. Let's look now at the new $999 Mac Classic. Basically, it's a fine machine. It works like a Macintosh is supposed to work. Its sound mechanism is excellent and you can connect all the devices you want to add via its six ports.

Be forewarned that it's slow, having the same 68000 chip as the Mac SE. (That's the Apple-world equivalent of IBM PCs and XTs.) There's nothing you can do about that. But if you expand the 1M RAM it comes with to the 4M RAM it can hold, it will run all of next year's Macintosh programs. That includes many firstrate teaching tools and games.

Cost for a Mac memory expansion board is $150. Cost to move to the 2M RAM we consider minimum in the Mac world is $149. A 40M hard-disk, 2M RAM Classic lists at $1,500. That's a bargain compared with the $2,100 PS/1 and the $1,900 TL/2.

But you may be disappointed once you get your bargain home. The Classic's 9-inch screen looks puny beside the 13-inch screens on the Tandy and IBM. And as with IBM type computers, many of today's Mac programs count on color. There is no way to add either a bigger monitor or a color screen to the Classic. If you want either (or both), you have to start with at least a Macintosh LC, which lists at $2,500 and up.

Now that you know what you'll really be paying, it's your choice. Any of these computers will do nicely for home use. But here's something to chew on. Tandy's long history of buying parts in volume and assembling low-cost computers has kept them in business for years. Apple's record for building workable cheap computers is, on the whole, pretty positive.

But every time IBM brought out a cheap computer, it left buyers in the lurch. Talk to anyone who bought an IBM Convertible (its first portable, now dead and gone) or IBM PCjr, also a goner!