You should clarify your question. It's somewhat difficult to understand.
I assume you mean should you buy an outboard preamp for recording. The answer depends on how much you want to spend.
If you're able to spend at least $500, then yes it MAY be beneficial for you to upgrade to a nicer preamp. If you want to spend less than $500 on a dedicated preamp, then I would advise you to stay with the Digi unit that you have (this is all assuming you have a Digi 002 or higher...since you didn't specify it's more difficult to answer your question).
I have the Digi 002 and the FMR RNP...I recently recorded drums with the RNP serving as the preamp for the 2 overheads, and the built-in pres on the 002 as the pres for the other channels. This worked out very well. If you have the 002 (or something similar) and you're planning on buying something like the DMP3, I would tell you to save up and wait.
Hopefully you can provide us with more information to help fully answer your question.