COMPUTER game makers have spoken about full-motion video on personal computers for a long time, as if it would revolutionise how PC-based entertainment worked. They are right, it would. Except that it has been unavailable on a ''personal'' level. Until not, that is. Sigma Designs of the United States last year released a video-cum-sound board that pretty much brings full-motion video to the desktop. Called ReelMagic, this add-on is now available in Hong Kong through Asia Concepts for $3,750. ReelMagic is nothing more than a card that sits on an IBM PC or a compatible's motherboard next to its VGA card and plays the new MPEG CD-ROM game format and the new VideoCD video format. MPEG, which stands for Motion Pictures Experts' Group, is a compression standard for video images that is already widespread in the PC market. MPEG achieves compression ratios of 200:1 by not only compressing data but by using ''intelligent guess work'' to cut out redundant data - such as those pertaining to recurring backgrounds in video sequences - from moving images. Capable of displaying up to 32,768 colours at resolutions of up to 1,024 pixels by 768 pixels at 30 frames a second, the card can make a PC the same if not better than a standard television for watching videos on. The standard television display rate is 30 frames a second, while ReelMagic's display resolution is better than a television's. What is exciting about the device is that the much-talked-about graphics-intensive computer games of today will look like an old Pac Man arcade game of 10 years ago in comparison with the quality of the games that will run through ReelMagic. With it, computer games billed as offering full-motion video will no longer look like low-resolution movie sequences of low-budget actors traipsing around in jerky slow motion. Computer games of today, at best, run at a display speed of 20 frames a second and at only 256 colours. James Henry, Asia Concepts managing director, said ReelMagic would allow PC users to watch CD videos on their computer screens - either full-screen or in a window on a side while working on a word processor, for example, in the main window. This is an interesting concept. The CompuTV from Packard Bell allows users to watch standard television on its monitor in much the same way. While the video CDs that ReelMagic will allow you to view on a PC are still a long way from commonplace, the first of them have been on the market for a few months. The video CD format is supported by movie makers Paramount Pictures and Warner Brothers and a consortium that includes Sony, JVC, Matsushita, Goldstar and Samsung. With this type of backing, fears that video CD may not catch on among PC users seem unfounded. ReelMagic's hardware needs are surprisingly low: at least a 386SX, 16-mega-Hertz PC with two megabytes of RAM and two MB of free hard-disk space. It can even work well with a one-speed CD-ROM drive. For software, it needs either MS DOS 5.0, or higher, or Windows 3.1, or higher. The card has a built-in 16-bit sound system that is said to be compatible with existing DOS and Windows software, as well as all the PC games around today. The sound card uses FM synthesis as does the more popular SoundBlaster card and, although it is not the best PC sound solution on the market, it does the job.