While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Alcoa (NYSE:AA) climbed 4% this morning after Goldman Sachs upgraded the aluminum giant from neutral to buy.

So what: Along with the upgrade, analyst Sal Tharani boosted his price target to $11 (from $8), representing about 20% worth of upside to yesterday's close. While momentum traders might be turned off by Alcoa's share plunge earlier this month -- fueled by concerns over aluminum price pressure -- Tharani believes Mr. Market is undervaluing the company's less price sensitive, high-margin segments.

Now what: Goldman expects Alcoa to add more than $2 billion of revenue and $525 million of EBITDA to its mid and downstream businesses over the next three years. "Alcoa's downstream businesses, particularly Engineered Products, are less aluminum price sensitive and we see upside in Alcoa despite our lackluster outlook for aluminum supply demand fundamentals," noted Goldman. "In addition, we see further upside beyond our current estimates from the automotive market through growth in 'body-in-white' body sheet, as well as from aluminum-lithium in aero." Of course, with the stock flirting with its 52-week highs today and trading at a forward P/E in the mid-20s, I'd wait for some of the excitement to fade before buying into that bullishness.