With its market cap at $213.9 billion, only two U.S. companies are bigger than Apple

Here’s one way to measure Apple’s AAPL progress in March, a month in which its stock price set 11 all-time highs.

On March 9, a few days after the company announced the iPad’s ship date, Apple’s market capitalization (share price times shares outstanding) was $198.5 billion, making it one of the five most valuable U.S. companies, right below Wal-Mart WMT and ahead of Berkshire Hathaway brk.a .

On Friday March 12, the day Apple began taking pre-orders for the iPad, its market cap hit $205.48 making it, briefly, the No. 3 U.S. company ahead of Wal-Mart — a position it held for just one trading day.

On Tuesday March 30, following a report that it was building an iPhone for Verizon, its stock rose 1.5% to $235.85, its market cap hit $213.9 billion, and it overtook Wal-Mart once again to begin closing in on No. 2 Microsoft MSFT . (See chart below the fold.)

Source: Wolfram Alpha

Two weeks ago, Apple was $50 billion behind its oldest frenemy. On Wednesday morning, the gap was $47 billion. To close it, Apple would have to reach a share price of $288.

[UPDATE: Apple was up 3.73 points (1.58%) in early trading Thursday following a crop of positive iPad reviews, reducing the gap to about $42 billion and the target Apple has to hit to catch up to Microsoft to $284 a share.]

That’s not out of the question. According to APPLinvestor‘s summary of analyst ratings, eight analysts have set targets of $280 or higher, and two of them — Credit Suisse’s Bill Shope and Citigroup’s Richard Gardner — are looking for Apple to hit $300 a share.

At $300 the company would have a market cap of $272 billion and be halfway to Exxon Mobil XOM .

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[Follow Philip Elmer-DeWitt on Twitter @philiped]