The sm7 is a wonderful mic no doubt, but be adivised it needs a fairly substantial high gain preamp to wake it up. It's a low volume dynamic mic. Even my Avalon is goosed pretty high to get a proper signal. A computer interface offering little to no additional gain won't cut it.

These are great little pres for short bucks. I have them, but have not used them with the sm7. Either one could do well, and the VTB-1 would be the better of the two all around, I believe.

http://pro-audio.musiciansfriend.com/product/Studio-Projects-VTB1-Tube-Blend-Mic-Preamp?sku=180360

http://pro-audio.musiciansfriend.com/product/ART-Tube-MP-Studio-Mic-Preamp?sku=180581

Dan