This is an amusing thread. One guy says it's all about the equipment, spend a few grand and you can do "professional vocals" in a broom closet. Another says it's all about skill, and with the right skills a $25 mic will get you "professional vocals". Another says it's all about the room, and yet another says it's all about having a "professional voice". Confused yet? I am.
I think the truth, as it so often does, falls somewhere in the middle. All of these things are important, but none of them exclusively. A great recordig happens when there is a great performance in a good sounding room, captured with good equipment that is handled with sufficient skills.
For vocals, the room is pretty easy since there's no low frequency energy, so just knock down the reflections with anything from blankets to Auralex.
For a microphone, it's not so much how much you spend, it's finding a mic that matches your voice. This might be a $1000 mic, or it might be a $100 mic ($25 ?.....I don't think so). It may take a few mic purchases to get there, or rentals, or book some studio time just to audition mics. This is really the hard part. Then you need to match it up with a mic pre that delivers your mic and voice at it's best. Oh, and one mic may be right for one song, but wrong for another, so a couple of mics with different characteristics is a good idea, usually one that's pretty neutral, and another that's a bit brighter and airy.
Skills are certainly important, though less so with vocal micing than drums for instance. During mixing though, plan on doing a lot of experimenting with compression, EQ, delay, reverb, etc. to find what works with each song. Read a lot, listen to your favorite vocals, and analyze what treatments they used to get thier sound. Many vocals are double tracked, with layers of backing tracks. Often vocals are comp'd, meaning several takes are done, and various best sections are cut and pasted to make one great take. Care must be taken to preserve continuity.
As far as the "professional voice", be realistic, but don't be dissuaded. Many great recordings have been done with less than stellar voices. The most important thing is that the vocal emotes in a way that is compelling. But if you want to sing like a proffesional, voice training and lessons are a very good idea.
Good luck, have fun.
- RD