Whether you'd hear the difference could depend on a lot of factors - including your AD/DA converters, monitors, room acoustics, your experience. But, yes, the RNP is a step up from the DMP3. The DMP3's pretty clean, and the RNP has a little color to it. Also a nice, extended high-end.
But if you can swing it, and really want a difference, then I'd be in the camp that'd say to save up and get a better mic pre. Average price for two channels of some of the good stuff is around $1500.
Of course it all depends on what you wanna do and what you're recording. You haven't given any info on that. Something like a $1200ish Peluso 22 251 is going to still sound pretty damn good, even through a DMP3. So, you might wanna consider how and where you want to spend your money to get where you're going.