There is no "system administrator" college degree...so you pretty much get to decide when you're equipped to be an IT consultant, and what your time is worth. Of course, repeat business is the only way you'll make decent money, so you'd not want to exaggerate your accomplishments. But, "serious Linux nerds" means people who are capable of setting up mail, DNS, web, users, LDAP/NIS/Samba, NFS/CIFS, and hooking it all up in such a way that it can operate without you present (I always used Webmin for this aspect--it has great ACLs so you can give as much or as little privilege as your customer needs to be able to create/manage users without breaking anything, for example).
Even if you're not an expert at all of these things, if you know what they're for and can read the documentation and make all of them work in an afternoon, then you're well-equipped (better equipped than many Microsoft "certified" techs) for the job. Most small businesses don't even know what they need, much less how to set it all up. So, when you go in, you'll find that they have no idea what you're doing, why you're doing it, or how it all fits together. How you explain it is more important than how you implement it when it comes to whether you're called back or recommended to other businesses. It has to work, of course, but as long as you're explaining the job clearly and in simple terms, the customer will remain happy (try to look busy, as well...sometimes you'll be waiting for installs and such...do something else during that time, even if it's busy work).
Note that I'm not necessarily recommending this path. IT support sucks. It's a very stressful field, where the only time you hear from people is when there's a problem. In fact, the only time you hear from people is when there is a problem so severe that they can't get work done (people are so afraid of computers, and of thinking about technology, that they'll ignore trouble signs until the breakage is complete). Thus, your customers are almost always stressed out. I frequently worked for a company that had a quarter million in daily revenues, and so a day with all of their people off-line cost them significant sums of money. I have great respect for the fellow who built and owned the place, as he kept his cool like a champ, no matter how tight the downtime schedules were...but some of his employees (and share holders) did not.
Oh, yeah, you'll also work a lot of nights and weekends, because that's when companies can afford to be completely off-line.