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Apple is widely expected to release a new version of the popular iPad tablet at a 1:00 p.m. EST event Wednesday in San Francisco -- and the new iPad 3 (or will it be called the "iPad HD"?) could go on sale in the U.S. as early as Fri., March 16.

Early speculation is of an evolved though not completely rethought tablet that has variously been called the iPad 3, iPad HD and iPad 2S. What is likely to appear is a model with a sharper, high-definition display that will likely be the focus of Apple's presentation. Early reports suggest the Retina-like HD display will pack four times the pixels (2,048 x 1,536) in the same 9.7-inch size.

This evolved and improved screen would make text look smoother and some high-resolution pictures look better. But it probably won't make much of a difference for images on the Web, or video. Still, the result should be absolutely gorgeous.

The other big rumor: 4G wireless speeds (that means LTE, for the acronym aware) that are likely to have some crying "at last!" In this respect, Apple is playing catch-up. Some competitors, such as Samsung and Motorola, already sell tablets compatible with these faster wireless networks.

The new iPad could also include Siri, the voice-activated "assistant" found on the iPhone 4S. Siri has gotten mixed reviews, but Apple has been touting the feature heavily in its advertising, and it would make sense to expand the availability of this high-profile feature.

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The new iPad could also benefit from better cameras, which would be simple enough to add.

But overall, the upgrade from the iPad 2 to the iPad 3 is likely to be less significant than the upgrade from the original iPad to the iPad 2, which added two cameras while cutting both the thickness and the weight of the device.

One big unknown is whether Apple will keep the iPad 2 in production and offer it at a lower price, like it kept the iPhone 3GS after the launch of the iPhone 4.

Another big question is whether Apple will reveal its rumored foray into making TV sets. Some have speculated that the invite to the Apple event, which said "We have something you really have to see," points in that direction.

Apple already sells an "Apple TV." It's not a TV, but a small box that attaches to a television set to display movies and play music from iTunes.

The iPad launch comes as Apple has reached a rare milestone: Last week, it was worth more than $500 billion. Only six other U.S. companies have been worth that much, and none have held that valuation for long. On Tuesday, Apple's stock fell, bringing its market value down to $493 billion, but analysts believe the company is worth closer to $550 billion.

Sarah Rotman Epps, an analyst for Forrester Research, said hardware features aren't that important to tablet buyers.

"It's about the services -- what you can do with the device," she said in a blog post.

Apple's competitors have slowly come to realize this, but only after bringing out dozens of tablets with whiz-bang features like 3-D cameras. The competitor that's done the best is Amazon.com Inc. Its Kindle Fire tablet is cheaper than the iPad, but what really sets it apart is that it's tied into Amazon's book, movie and music stores, making it an easy route to entertainment, just like the iPad.

Still the Kindle Fire has a long way to go. Epps estimates that Amazon sold 5.5 million Kindle Fires in the fourth quarter of last year. Meanwhile, Apple sold 15.4 million iPads, and has sold 55.3 million in total.

According to Canaccord Genuity, 63 percent of the tablets shipped last year were iPads. The only competitors with more than 5 percent market share were Amazon and Samsung.

Stick with FoxNews.com or follow us on Twitter for all the details as Apple makes its big announcement.

The Associated Press contributed to this report.