Apple Macs accounted for 66% of all the personal computers sold for more than $1,000 in U.S. retail stores during the first three months of the year, reflecting the company's ability to tap into affluent households with multiple PCs, a market research firm said Tuesday.

The latest retail numbers from the NPD Group include only store sales, including Apple stores. They do not take into account online sales, a major channel for PCs. Dell, the world's second-largest computer maker, sells the majority of its PCs online.

Nevertheless, the number is impressive. "It means Apple is able to sell stuff at a price point and quality that other OEMs [manufacturers] and retailers aren't able to do," NPD analyst Stephen Baker said.

A major contributor to Apple's success is its ability to tap a lucrative part of the consumer market, namely, affluent households adding more PCs than the typical one or two found in homes, Baker said. In addition, Apple has done a good job in terms of marketing to get people into its own stores, where the choices are limited to Apple products. If people go to Best Buy, Circuit City, or some other retailer, they're confronted with multiple products spanning a wide range of prices.

"There's a lot of competition in other stores, but there's none at the Apple store," Baker said. NPD wouldn't release numbers on Apple sales through it stores, but Baker said, "it's a pretty substantial number."

Put it all together and Apple Mac sales are very strong. Asked if he thought Apple was firing on all four cylinders, Baker responded, "If it was just four, they'd be a Yugo. It's eight cylinders. They're a great, big Hummer."

In April, Apple reported it shipped 2.3 million Macs in the first quarter of the year, a 51% increase over the same period a year ago. Revenue from its computers rose by 54%.

Apple this month opened its largest U.S. store. The Boston store features three floors, one for Mac laptops and desktops, the second for iPod music players, and the third for service.