Whenever replacing the engine mounts on front wheel drive vehicles, you should always replace them all at the same time. This is for several reasons. Two of the mounts on that car, hold the weight of the engine (support the engine from the car body). As the vehicle ages, those mounts slowly collaspe down. This causes the two mounts that control the engine movement to streach and allow more engine movement then should be allowed. This allows parts of the engine (mainly the Exhaust system) to come into contract with the body of the vehicle and create a lot of noise.
Also as the supporting mounts collaspe, they no longer are isolating the engine from the body and allowing a considerable amount of engine vibration into the body, again causing a considerable amount of noise.
The reason that this is key on a front wheel drive vehicle (and less so on a rear wheel drive vehicle) is because the entire drive package is supported from those points. They not only have to support the engine and transmission, but are also used to control movement from the power package transmitting power to the ground.
On a rear wheel drive vehicle, you will have (at least) three mounts holding the power package to the body, but none of them have to control as much engine movement, as the power is applied throught the rear differential.
Also make sure that the lower rear mount is clean and free of rocks and dirt. This is one point that tends to get contaminated due to its location.