while your mic locker will cover your basic needs, i recommend getting that sm7 or ev re20, plus a decent tube lcd BEFORE investing into another preamp.
people on budget - RNP is good. sytek too. unless you're auditioning the entire mix (10+ tracks at once) you will not notice any improvement over your on-board pre's.
are you digital?
if you're going thru some kind of converter and back, it better be a good one - otherwise you should stick with your board pre's.
this is to tell you that no matter how great the improvement of your pre's - what you put into it (mic) and the path before hitting its final destination (converters, processing, etc.) can be even more important.
good luck