* Cook expounds on post-PC world

* Upgraded Apple TV still $99

By Poornima Gupta and Alexei Oreskovic

SAN FRANCISCO, March 7 (Reuters) - Apple Inc took the wraps off a faster 4G-equipped iPad starting at $629 , hoping the latest version of its tablet can safeguard its dominance as rivals from Amazon.com Inc to Samsung Electronics pile into the market.

Chief Executive Tim Cook, presiding over his second major product launch since 2011's voice-enabled iPhone 4S, introduced the highly anticipated third iteration of the tablet, which is available for pre-orders from Wednesday and will hit store shelves March 16.

The high-end model of Apple's latest iPa d will be capable of operating on a high-speed 4G "LTE," or Long-Term Evolution, network. At speeds roughly 10 times faster than current 3G technology, that may help banish the sometimes shaky video quality of older devices.

Wall Street had anticipated many of the features Cook enumerated on Wednesday, including a higher-definition "retina display" and a better camera.

"The iPhone 4S showed us that Apple doesn't need to out-do itself with new product designs to continue extending its domination of a category," said CCs Insight analyst John Jackson.

Apple is betting a 4G-equipped iPad will tempt more U.S. consumers to pay extra for higher-quality video on the go. That, in turn, should give Verizon Wireless and AT&T Inc a revenue boost, analysts say.

Until now, buyers have been reluctant to shell out extra cash even for iPads with slower 3G connections. The cheaper Wi-Fi-only model -- with much more limited Web access -- is by far Apple's top-selling one today.

An updated version of the Wi-Fi-only model remains at $499 . The most expensive 4G model, with 64 gigabytes of storage, will go for $829.

The company is counting on a warm reception to its latest tablet to fend off an increasingly aggressive challenge from Google Inc Android-powered tablets, with Microsoft Corp software-driven devices slated to come soon.

"Everyone's been wondering who will come out with a product that's more amazing that the iPad 2," Cook said.

"Stop wondering: we are."

WHAT'S THE BADDEST TABLET ?

Shares of Apple, which had trended upward by as much as 1 percent in morning trade, dipped briefly into negative territory as the iPad was unveiled, following a familiar pattern established at previous launches.

Earlier in the session, Cook again held forth on what he called a "post-PC world," in which users move increasingly away from traditional desktop and laptop computing and toward an array of portable devices -- including tablets.

Smartphones and tablets are start ting to eat into PC sales as mobile technology gets more advanced and available content expands.

Some experts believe mobile devices, as they get more powerful, will eventually displace PCs in many markets, hurting business for the likes of Hewlett Packard and Dell .

The global tablet user base reached 67 million in 2011, according to researcher Strategy Analytics. Analysts expect double-digit growth in general of tablet sales in coming years.

Cook also announced that the company's new $99 Apple TV set-top box, a concept that late CEO Steve Jobs had called a "hobby," now supports high-definition 1080p screen technology.

"Last year alone we sold 172 million post-PC devices," Cook told the audience at the Yerba Buena Center in downtown San Francisco, Apple's preferred venue for product unveilings.

"And this made up 76 percent of our revenues. This is incredible."

• A Permanent Economic Emergency