Private equity firms are flexing their muscle this year like never before, and that's good news for Wall Street investment houses faced with a stock market that's having a hard time gaining traction.The importance of private equity to Wall Street was on display Monday with the $33 billion leveraged buyout of HCA ( HCA), the Nashville, Tenn.-based hospital chain. Along with the enormous fees the deal is generating for a slew of investment banks, one Wall Street firm, Merrill Lynch ( MER), stands to also benefit as a principal in the transaction. Merrill Lynch, along with well-known private equity firms Bain Capital and Kohlberg Kravitz Roberts, are the big-money investors agreeing to buy HCA for $51 a share, an 18% premium to HCA's price before news of the talks emerged last week. The deal calls for the trio to pony up about $6 billion, while raising $15 billion in bank loans and bonds. The deal includes HCA's $11.7 billion in assumed debt. Look for Merrill Lynch and other investment banks to do their best to keep the go-go market for private equity humming. It's the kind of high-margin business Wall Street needs in an increasingly stagnant market. "The private equity market has definitely taken off, and these big deals are becoming more common," says Matthew Rhodes-Kropf, a professor at Columbia University's business school. In fact, the total dollar value of buyouts this year has surpassed last year's record-setting figure, according to Dealogic. Private equity firms usually make money by buying a company and cutting costs or reforming the business over a number of years. Depending on the investment, the private equity firm often will pay itself a dividend each year for its work, then sell the restructured business for more than the purchase price.