The preamp would plug into the TRS (Line in) input on your mixer, that way it would not go through the built in preamp. Preamps, even expensive ones are unfortunately not magic devices that make everything sound awesome, so no, at this point you do not need one.
Incidently, if they ever do invent a magical device that makes everything sound awesome, I will sell my car and mortgage my house to buy it, until then I will have to continue to work on my chops.
I think there was a time when some people thought Autotune was such a device...
So what does a better preamp do:
Less than you would hope! It does less than a better mic would do, and much less than proper mic technique does.
Cheap preamps tend to make this sound a little bit mooshy and noisy compared to the really nice stuff.
Please note that "mooshy and noisey" can also be caused by alot of other things/techniques in your setup.
I am not trying to patronize you my friend, I am merely trying to help you avoid the pitfalls of gear lust AKA GAS!