There is no single correct answer to this -- every answer will be subjective, and your options will change dramatically depending on your price point. Every microphone and mic preamp has its own character, and it's up to you to determine whether that character is what you want for that application.
The previous poster is correct -- the Shure SM 57 is the Swiss Army Knife of microphones. If your budget is under $100, you can pick up a 57 and use it on just about everything. It won't always be the best mic for the job, but it will record anything from a screaming half stack to classical guitar. They are famously durable, a trait which is not necessarily shared by high-end condenser mics.
If you go to a recording studio, you will most commonly see acoustic guitar recorded with a high-quality small diaphragm condenser microphone.
As for whether you should go for a matched pair of small diaphragm condensers, this will also depend on both your budget and the specifics of the application. If the acoustic guitar is the foundation of a song, you may find it useful to record in stereo. If you record in stereo, you will likely find that you get better results with a matched pair of small diaphragm condensers.
There are more threads on recording boards about which small-diaphragm condenser mics are the best for the money than one could ever reasonably read, so the best thing for you to do is go somewhere where you can test several options with your guitar, and pick what sounds best to you.
There are some people out there who work miracles modifying cheap microphones. Mark Fouxmann can take an AKG C-1000, which you can find for under $100 used, and make it sound like a mic that costs ten times that much.
I have one of these modded C-1000s, and I've A/B'ed it in a recording studio against some very high-end mics, and been very impressed with the results.
In regards to placement, this is again subjective. The default placement is around the 12th fret, away from the soundhole. You may find, however, that there are other placements you like for different applications. Try having a friend play your guitar while you listen. Move your ears to different spots, and find what sounds good to you.
There will never be a single right or wrong answer. Know your options, experiment, and trust your ears.