Should I buy an audio interface or tube pre-amp to record vocals?

I want to record vocals with my AT 4040 mic and currently have a very cheap preamp. Am I better off spending my money on something like the Presonus Firebox (which has only "decent" preamps), or buy a higher quality tube pre-amp for a few hundred bucks and go digital right into my computer (i have a sound blaster audigy card with a bunch of different inputs)? I use Steinberg LE to record but don't know what other factors might come into play. Thanks for any help you can provide! (Also, any good tube pre-amp recommendations are welcome).