Amid Day-After Jitters, Opportunity Beckons
Investors, from major corporations to individual stockholders, remain unfazed by stock-market plunge, regarding it as opportunity to shop for bargains; day after record 554.26-point drop, Dow Jones Industrial Average soared 337.17 points, greatest single-day point gain ever from investor share-shopping spree in which record 1.2 billion shares changed hands; photos (M)