(FORTUNE Magazine) – In case you haven't heard -- and if you haven't you may qualify for The Guinness Book of World Records -- IBM is due to unveil a new line of personal computers in the first week of April. Anticipation has been helping to boost the stock price, but investors had better watch out. When IBM's competitors in the PC market made similar product announcements earlier this year, their shares rose to magnificent, but unsustainable heights. In the six weeks before Compaq Computer's introduction of its new Portable III transportable computer in February, the company's stock was up 70%, from $20 a share to $34. Amidst smoke, lights, and a bagful of magic tricks at the product's announcement in New York's Mark Hellinger Theater, Compaq Chairman Ben Rosen spoke with glee of his company's newfound strength on Wall Street. But the Portable III contained few unexpected features, and Compaq stock has since fallen to a recent $29. Apple Computer shares took a similar ride. The stock rose from $41 a share at the beginning of January to $70 the day before the long-awaited March introduction of the Macintosh SE and Macintosh II. Within two days Apple stock fell $5 a share; it had recently recovered to $68. What should investors expect from Big Blue? The new IBM machines will probably include bargain-basement PCs aimed at students, priced low enough to compete with the cheapest of the PC clones; a redesigned PC-AT; and a machine incorporating Intel Corp.'s speedy 80386 microprocessor. IBM shares have risen about $25 since January 1, and even though personal computers contribute only around 10% of revenues and profits, the impending introductions seem to have added fuel to the stock's advance. ''The PC announcement has a psychological life of its own,'' cautions computer industry consultant Ulric Weil. ''People hype themselves into euphoria. Then reality takes over.'' Investors might watch for a bit of reality the week of April 13, when IBM is expected to release unflattering financial results for the first three months of 1987.