I'm going to jump in and be different. I have a very similar setup to you (SBLive, SM58, not much else), and I looked at the pre-amp option, but ended up buying a beaten up second-hand
12 channel mixer (Studiomaster Diamond 12-2. So old that the studiomaster site refuses to acknowledge it exists). My reasoning here was:
a) preamps in general cost more here than in the US , so $100 suddenly is closer to $400, and
b) while I only wanted to record guitar and piano when I first started, having 12 channels meant that recording a drum kit live wasn't a problem (ie. mixing straight to a stereo track using my 58 and three $20 mics).
c) when my mates come round, I can mix the guitars, vox etc and even out the sound somewhat, and run all the output through the one decent amp we have between us (but this reason probably doesn't affect you so much)
It all depends on what you want - versatility (mixer) vs. quality (Art Preamp).
Eventually I'll buy a preamp, but with an SBLive, any joy gained with the preamp was soon distorted to hell by the SBLive.
- gaffa