Here’s something you probably haven’t thought about in a while: What does it mean for stocks if the U.S. dollar actually goes up? The greenback has exhibited strength in the past few days, putting it close to a breakout above both its 200-day moving average and the range it has been mired in since mid-March.

As would be expected, the euro is in the opposite predicament. The nearby 140 level represents both support and the approximate location of the 200-day moving average:

A stronger dollar isn’t a factor investors have had to consider for quite some time, but now might be the time to put it on your radar screen. The weak dollar generally has been a positive for the U.S. market, since it has boosted the earnings for the large — and growing — number of companies that generate the majority of their revenues overseas. Conversely, dollar strength could knock out a pillar of support for domestic large-cap stocks. The table below, which was sourced from a recent Standard & Poor’s report, shows estimates for the overseas revenues for the various S&P sectors.

These numbers probably understate the actual totals, since only about two-thirds of S&P 500 companies provide a geographic breakdown. Also, the methodology limits the universe to companies that have no less than 15%, and no more than 85%, of their revenues from overseas. Still, this table provides a gauge of just how important non-dollar earnings have been for U.S. companies.

Taking this a step further, the Toronto Globe and Mail reported the following tidbit in May: “CIBC World Markets chief strategist Peter Gibson noted recently that there are roughly 80 to 100 companies on the S&P 500 that derive more than half their revenues from outside the United States — and these companies accounted for virtually all the profit growth on the index in the past year.” Further, S&P 500 companies reported sales growth of 11.4% in 2010, well above the 2.9% rise in GDP. Add it up, and you can see the importance of overseas earnings — and by extension, the danger of a stronger dollar and its potential impact on exports.

So what’s the actual impact on performance? Recently, it has been modest but noticeable. In the interval from April 29 to May 23, 2011, the dollar rose about 3.8% and the S&P 500 declined roughly 3.9%. From Nov. 4 through Nov. 30, 2010, the dollar climbed about 6.5% and the S&P fell 5.0%. The relationship between the U.S. Dollar Index and the S&P 500 is expressed in the chart below:

The takeaway, for investors, is that it’s time to start thinking about the potential impact of a rising dollar on the future earnings of the companies you own. The effect of a rising dollar would be particularly strong on global companies such as Phillip Morris International (NYSE:PM), Yum! Brands (NYSE:YUM) or Heinz (NYSE:HNZ). The tech sector also carries an outsized weighting in companies that rely on overseas sales to drive growth, including Qualcomm (NASDAQ:QCOM) and Intel (NASDAQ:INTC). Commodity-related shares also stand to lose in a rising-dollar environment. In the April 29-May 23 interval mentioned previously, when the S&P fell 3.9%, the Materials Select Sector SPDR (NYSE:XLB) fell 6.7%.

This S&P report from last year, while dated, provides data regarding the overseas revenues of various large-cap companies.

Also at risk are companies that have experienced rapid recent growth in their overseas revenues. A list, together with the full Globe and Mail article on the dollar, is available here.

Investors also should keep an eye on the Australian dollar, which has displayed a tight correlation with the U.S. stock market in both the recent past and over longer-term time periods. The Aussie, as represented here by the CurrencyShares Australian Dollar Trust (NYSE:FXA) sits 3.6% above its 200-day moving average. A breakdown could be a signal that further trouble is ahead of the U.S. market.

Given the extended period of weakness in the dollar, the burden of proof is on the greenback to show that it can sustain an upward move. Still, with the situation in the eurozone growing more perilous, the outlook for the dollar is better than it has been in quite some time. Keep a close eye on currency movements in the weeks ahead to gauge the future direction of stock prices.