That card doesn't have an XLR input, which is what the SM58 is optimized for. What you *could* do is buy a mic preamp with phantom power. You can then plug either the SM58 or any condenser mic of your choice into that, and run the preamp line outs into your X-Fi to use its nice analog-to-digital converters.
This way you're not wasting anything, and getting best use of everything. Mic preamps go from cheap crap to ridiculously expensive. The best under $200 is probably the Studio Projects VTB1 - http://bit.ly/vCsIe4 This will open you up to being able to use nearly any mic with your set up, 24-bit.
Also the mics don't care what bit rate you use; that all depends on your audio interface a/d converters. The AT2020USB only does 16 bits because the USB a/d converter inside is limited to that, to keep the price down. If it were a regular XLR pin condenser, you could plug it into a million bit interface if such a thing existed, without problems.
About the SM58 ... it's a dynamic designed more for live use. For recording, it'll work and on some voices better than others. It may work for you.
Don't forget to use compression and subtle EQ on your vocals to bring out their best, especially when using a dynamic. A condenser is more sensitive and picks up sound from a wider angle, therefore it sounds more open, airy, natural whereas an SM58 can sound more up close and potentially stuffy. Advantage is that an SM58 doesn't pick up as much background noise.
As far as dynamics go, one of the best is the Shure SM7B but it's around $300-$350. For condensers, the AT2020 is a low end work horse that works on most things but needs some reduction in the treble region to sit well in a mix. For clarity, detail, and openness the AT2020 beats the SM58 hands down, but like any condenser, you do need to hang up blankets or sound treatment in your studio to get the best vocal sound.
The higher end AT40407 or AT4050 get lots of respect though, as do the AKG 414 and AKG 214 mics. Also look into the Kel HM7u for a nice sounding but pricey mic, or the Kel HM2U for a smooth SM7B alternative in condenser form. That's for down the line if you want to upgrade your mics.
If you opt for the AT2020USB, to be honest you won't notice a big diff between 16 bit and 24 bit recording, and you will get clear sounding vocals. Just that the preamp+mic combo discussed above gives you more options down the line.
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