Just as they did in 1984, with the launch of the Macintosh computer, and again in 1998, with the introduction of the iMac, Apple Computer has decisively and with great panache redrawn the world concept of what a computer should be.

At San Francisco's MacWorld Expo on Tuesday, a stunning new line of powerful, translucent Macintosh desktop computers based on Apple's G3 chip was triumphantly unveiled during a much anticipated keynote by Mr Steve Jobs, Apple's "interim" chief executive officer - or iCEO, as he termed himself to the audience, in a play on the name of the iMac.

Well-known for springing surprises on MacWorld audiences, Mr Jobs also launched the 1999 version of Apple's highly successful, low-end consumer Macintosh, the iMac.

Not only has it been upgraded to a faster 266 MHz G3 chip and the price pulled down slightly by $100 (#86) in the US, but in a move the company managed to keep secret until the announcement, the "Bondi blue" casing of the iMac will now come in five colours.

"They are beautiful and we hope people will like to collect them," quipped Mr Jobs, who is using the Rolling Stones song She Comes in Colours, to market the machine. "We think one of the most important questions when you buy a computer now is going to be, "what colour do you like?"

For those more concerned with specifications and performance, though, the new G3 Power PC line marks Apple's serious bid to be seen as the consumer machine of choice.

For many years, Macintoshes were strong on ease of use and design but lacked power and speed and only had a limited capacity for upgrading. Now, according to Mr Jobs: "We want to make not only the most powerful Macintosh ever built, but the best PC in the industry."

The most muscular of the new range has a 400MHz G3 computer chip, and all come with 64 megabytes of internal memory, a six gigabit hard drive as standard, upgradeable to a whopping 100G, and, in an interesting move, in-built 3D graphics acceleration with 16 Mb of memory as standard. It costs from $1,599 to $2,999 in the US.

The G3s also feature a simple innovation which drew gasps from the crowd of 5,000 - a door on the side of the G3 tower which pulls open and drops down, allowing easy access to all the computer's components. While millions of computer owners routinely perform system upgrades themselves, no computer maker in the 20-year-old consumer industry has managed to incorporate such an obvious detail.

What the technical jargon boils down to is a range of fast machines in which both internal memory and storage space can be expanded to accommodate even the most demanding users. Some of those will come from Apple's strongest and most loyal customer base - those who work in the graphics and publishing industries.

But such high specifications and attention to 3D capabilities are clearly designed to make the new G3s a tempting and cost-efficient games machine - equivalent graphics accelerator cards for PCs must be bought separately and cost about $200. For consumers, the ability of a computer to run games, and the wide availability of gaming titles for a particular operating system, has become a crucial factor in the success of most home systems.

"We recommitted Apple to games nine months ago and we've been working really hard at this," said Mr Jobs. "Now, we want to be the best gaming platform in the world." He noted that 12 of the leading computer games, including Tomb Raider III and Quake, will be released on the Mac in coming months.

Mr Jobs also trotted out Mr John Carmack, the co-founder of leading games company iD Software, who promised that the much-anticipated game Quake Arena would be released simultaneously to the Mac and Windows platforms.

An additional coup for Mr Jobs was the announcement that software developer Connectix, which sells the popular VirtualPC Mac program which allows a Mac to run Windows applications, also has released a $49 program this week that enables the Mac to run any game created for the Sony PlayStation. Sales of the program were brisk at Connectix's crowded stand in the exhibition hall.

Mr Jobs also announced the release of a completely rewritten version of the Macintosh operating system. Mac OS X for servers incorporates features - preemptive multi-tasking and protected memory, which allow a computer system to run more efficiently, at higher speeds - that Apple needs for a truly modern, muscular OS. The consumer version of OS X is due out around the third quarter of this year.

San Francisco's annual MacWorld Expo is the largest Mac event in the world and is considered the most important, because the company announces its new products and direction for the coming year.

Because Apple has been ailing for so many years, with a particularly steep slide in revenue and market share between 1995 and 1997, MacWorld routinely garners more attention than any equivalent event in the computing world.

As Apple's market share slowly waned from 18 per cent to about 3 per cent by the middle of last year, many journalists considered themselves to be on deathwatch for the company. But in a surprise move, Apple co-founder Mr Jobs returned to the company as a "special adviser" to then-chairman and CEO, Mr Gil Amelio, at the start of 1997.

Later that year, in an apparent overthrow, Mr Amelio resigned and Mr Jobs took the job he has retained since then, as "interim" CEO. He receives no salary for the position and owns one share of Apple stock.

But the mercurial Mr Jobs, who once, at age 12, rang up Mr William Hewlett of computer company Hewlett-Packard, to ask for some computer parts - which he received, as well as a summer job offer - has gradually rebuilt and re-invigorated the company.

He's managed the task despite doubts that he would be able to balance leadership with Apple with his job as head of animation company Pixar, creator of the films Toy Story and A Bug's Life.

The release last August of the phenomenally successful iMac, an inexpensive computer designed to enable consumers to link easily to the Internet, has hastened the firm's recovery. The iMac is now the best-selling retail PC in the US, and Mr Jobs noted that by year's end 800,000 had been purchased, or one every 15 seconds.

Importantly, 32 per cent of buyers are first-time buyers, and 13 per cent are converts from the dominant Windows/Intel platform. Apple's market share has risen to 5 per cent and the company will report a fifth straight profitable quarter - a turnaround many believed they would never see - but the company will need many more Mac newcomers to continue to build significant customer numbers.