Will is correct!
We need more info.
1) What are you trying to record? Vocals, flute, tuba?
What works best for the twelve-string on "Over the Hills and Far Away" ain't necessarily gonna be the best setup for the drums on "When the Levee Breaks".
2) What type of music? Duane Allman's "Little Martha" and Richie Haven's Woodstock version of "Motherless Child" were both acoustic guitar, yet the sound is very different.
3) What is it about your current setup that you're dissatisfied with, or are trying to improve upon? This is important in your situation because it will determine where you want to start.
If you're trying to 'clean up' your sound, feeling that it's too distorted, or noisy, then I would suggest moving up to a low-end 24-bit sound card, like an M-Audio Audiophile, and a clean, cheap preamp like an M-Audio Audio Buddy. (No, I don't work for M-Audio!)
If, on the other hand, you're realtively happy with the fidelity of your sound and your signal-to-noise ratio, but you're looking to add some 'life' and/or 'sparkle' to your vocals, then I agree that the mic/preamp combo would be where you want to focus.
If you specified roughly what your budget would be, that would help, too.
The Audiophile runs about $150-180, the Audio Buddy $80-100.