You're generally better off recording your tracks raw/dry and doing any "juicing up" in the mix. Once you've recorded a track with an effect, it's unchangeable even if you decide something else would sound better when you hear the whole mix together.
As for separate pre amps, my personal view is that they're not worth it unless you've spent a LOT of money on the mic and are willing to spend an equally big "lot of money" on the pre amp with a specific goal in mind in terms of how you wish to vary the sound. The internal pre amps in most interfaces are generally pretty neutral and useable. Others have different ideas of course.
The pessimist sees the glass as half empty. The optimist sees it as half full. The realist just drains the darn thing and gets a refill!