United Rentals Inc., Greenwich, CT, has filed a lawsuit against Cerberus Capital Management's RAM Holdings Inc. and RAM Acquisition Corp. Cerberus had agreed to acquire United Rentals, but recently Cerberus has second-guessed its decision to do so.   ; The lawsuit wants to force the agreed-upon acquisition, saying Cerberus has violated the merger agreement and should not be able to avoid the agreed-upon transaction by paying a break-up fee. The lawsuit contends that Cerberus has the financing necessary to complete the transaction.   ; United Rentals is an equipment rental company with a network of more than 690 rental locations in 48 states, 10 Canadian provinces and one location in Mexico. The company offers for rent over 20,000 classes of rental equipment with a total original cost of $4.3 billion.