My latest column is up at RealMoney.

With the stock market flirting with “official” bear market territory, I realized I hadn’t written a bearish piece in a couple of months. Not wanting to buck the trend any longer, I decided to look through the models I follow to see which stocks might be on the pricey side. I think I found one in ACI Worldwide (ACIW) .

ACI develops, markets, installs and supports a broad line of software products and services primarily focused on facilitating electronic payments. The company’s products and services compete with offerings by Fiserv (FISV - Annual Report) , Fidelity National Information Systems (FIS) , S1 Corporation (SONE) , Metavante (MV) , Euronet (EEFT) , Fair Isaac (FIC) , Visa (V) and MasterCard (MA) .

About the only argument one can make in favor of a long position is that the stock has come down a lot — nearly 50% from last July’s peak. Unfortunately, at last July’s peak it had already come down a lot from the prior year’s peak. It is amazing to me that a stock performing so poorly can still be valued as highly as it is. For now, I’m not counting on a reversal in price momentum.

For those of ACI Worldwide’s peers that have earnings on which to base a P/E multiple, the average P/E is about 16. At 16 times the 58-cent current 2009 consensus estimate for ACI Worldwide, the stock would trade at just $9.28 — 46% below the current level. Even at the 21 times multiple S1 enjoys, the downside could be 30%. And those prices assume the company will actually earn what analysts believe it will. As noted earlier, that has not been a safe bet of late.

Disclosure: At time of publication, William Trent has no financial position in the companies mentioned in this article.