Tooleyweed

I would say that a pre-amp and a compressor are a must. Having a good vocal compressor that is hardware driven and not software driven will be one of the best investments you can make.

By placing a compressor between you mic and your DAW you can lay down vocal tracks that aren't clipping and that have a full saturation.

If you are planning an making pro-quality vocals than you will also need a decent mic and not an $89 mic from Radio Shack.

Not as important, but very very useful is having a parametric equalizer...this can be in software format and will allow you to control things like "presence" and "breathyness".