Congress approved landmark legislation today that opens the door for a new era on Wall Street in which commercial banks, securities houses and insurers will find it easier and cheaper to enter one another's businesses. The measure, considered by many the most important banking legislation in 66 years, was approved in the Senate by a vote of 90 to 8 and in the House tonight by 362 to 57. The bill will now be sent to the president, who is expected to sign it, aides said. ''Today Congress voted to update the rules that have governed financial services since the Great Depression and replace them with a system for the 21st century,'' Treasury Secretary Lawrence H. Summers said. ''This historic legislation will better enable American companies to compete in the new economy.'' The decision to repeal the Glass-Steagall Act of 1933 provoked dire warnings from a handful of dissenters that the deregulation of Wall Street would someday wreak havoc on the nation's financial system. The original idea behind Glass-Steagall was that separation between bankers and brokers would reduce the potential conflicts of interest that were thought to have contributed to the speculative stock frenzy before the Depression. Consumer groups and civil rights advocates criticized the legislation for being a sop to the nation's biggest financial institutions. The opponents of the measure ... predicted that by unshackling banks and enabling them to move more freely into new kinds of financial activities, the new law could lead to an economic crisis down the road when the marketplace is no longer growing briskly.