I have literally no experience with the mixer that you have, but my guess is that an outboard pre-amp would be better. I like to think of pre-amps as the "sparkle" of a recording. I haven't found that "budget" pre-amps give a night and day difference in terms of the quality of recording...but if you listen to your stuff, and then listen to stuff that was recorded in a "real" studio, and the pro stuff just has more life, or more sparkle....well, there are alot of reasons for that probably (I'm just a home recorder, after all) - but I've found that I can get more of that intangible sparkle with better pre-amps. I went from Behringer board pre's to a Blue Tube, to a DMP3, to a Tampa. I don't see myself getting anything better than the Tampa, it makes my SM-58 sound really nice.
Just my thoughts on the subject. I know that we all get caught up in wondering whether an upgrade will really be better or not (that's how I ran through all the gear above....)
There are lots of discussions about this, do a search on RNP or DMP3 - all the threads will probably be something like, "will getting a RNP really improve my sound?"