The next most important thing, after trusting what you hear, is to trust your recording chain. This means mic>cable>preamp>converter>recording software (REAPER, presumably).
Notice that I said "trust" is the most important thing. That is, it is more important to trust it than to have it be a great one. If this seems counter-intuitive, it is. More time and money is wasted by home recordists second-guessing their gear and wondering whether the preamp or whatever is good enough than anything else. If these people simply trusted that what they had could work, and focused confidently on technique, they would achieve more in an hour towards improving their recordings than by spending months reading reviews and forums and how-to books.
So if you have any doubts about the ability of your gear to capture good recordings, try this test (suggested by the brilliant Ethan Winer in this month's Tape Op):
Take a great-sounding CD and record it through your soundcard. Play back the recording. If it still sounds great, then you know that your soundcard is capable of rendering great-sounding recordings. No more blaming the interface.*
Next take the same CD and play it back through your monitors, recording the playback with your favorite mic (this is actually how the earliest records were duplicated). Still sound good? No more blaming the mic, cable, or preamp. If it doesn't sound good, then go back to the above post and make sure that your monitors and room acoustics are up to snuff. Even the lowly SM57 should reproduce a pretty accurate picture of whatever you point it at.
If you cannot get a good capture with what you have, then it's time to try and wring out the signal chain for the weakest link. But since I suspect that most home recording rigs will more or less pass this test, I'm going to set that part aside for later.
*Please note that none of this is to say that preamps or converters or mics don't matter. Better tools make things easier. But merely adequate tools can still build a great project. The pyramids of Egypt, the Taj Mahal, Buckingham Palace, and John Hammond's brilliant recordings of the Benny Goodman Orchestra were all created without tools that modern craftsman take for granted.