If you just got Logic and the Duet, congrats. This is a great combo and you can get killer sound from it. Start there and ask yourself whether you are happy with what the Duet is doing for you - if you are your money is best spent on advancing your mic collection, monitors or headphones, maybe a nice compressor (RNC is cheap AND good...) - good cables if you don't have them yet - Mogami, Monster, etc. You've got a good 2 channel pre and AD/DA device there, make sure the rest of your recording chain is up to snuff.
But let's say you DO want that something extra in the mic pre category. Do you like the clean pre sound you get from the Duet? Go for a "colorful" pre like a Chandler germanium (surf gearslutz.com for more info on what's what - once you get into "colorful" there's a lot of different colors). Don't like the Duet's pre sound, want an all-rounder? The Great River MP 1NV is a great choice (and at 1K for one channel it should be). Does pretty clean to pretty colored, very versatile - but remember that it's only one channel of pre for ~1000 bucks, and depending on what you're recording you may want to do stereo. Still, if you want that for the meaty RAWK guitars and in your face vocals, you can do the stereo acoustics with your clean pre (the Duet).
Just remember that you're only as good as your weakest link in the chain, and do your homework (start by surfing gearslutz) if you're really plunging into recording. It's an expensive hobby, invest wisely.