Here's a cheap one, not great, but it's going to be my first condenser specifically for this purpose. When I get an acoustic that is. It's cool that it has a 10dB pad and 80Hz roll off at this price point.

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My first idea is that you should experiment a good deal more with mic placement and the 57. Where exactly have you tried positioning the mic? And besides volume try and define what tone you are lacking.

My next thought is that you might want to try dual micing with 57's before buying the condenser that GF recommended, which from his post I am not certain that he has used for acoustic guitar.
One of the issues that I have had with using LDC (large diaphragm condenser) is that you pick up so much of the room. If you have a really great room acoustically then this is an added benefit but if not it can make things worse. You will again, have to experiment a good deal with placement including where you sit in the room, small changes can make a huge difference.

Check out this thread, there is a post by Bob Rogers that entails the best spot to place a mic within a room. You can apply this to acoustic guitar or anything for that matter.

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This might also help if you have more than one mic in your collection.

http://audio.tutsplus.com/tutorials/recording/6-stereo-miking-techniques-you-can-use-today/

OK, here is something to think about. While I too love, simply love the SM57, you may enjoy the quality sound from your Radio Shaft condenser microphone. And here's why. While you might find many cool condenser microphones today that hereto for were not available in the past, are all made possible by the same people in Taiwan, making Radio Shaft microphones. It's the capsule that counts. Case in point. I have 2 Crown PZM microphones that cost approximately $375 each. 19 years ago, Crown licensed its PZM microphones technology to Radio Shaft. And you could buy the Radio Shaft PZM version for $30! Of course, you could not phantom power the microphone. It required a double-A battery. And it was not balanced out as it only had a 1/4" mono plug. It didn't take folks long to find out that it actually had a balanced output transformer. You only needed to remove the 1/4" connector to find red, white & ground wires. Easily wired to a balanced XLR connector. Then, you could find these EverReady alkaline batteries that were six volts & half the size of a double A. So you would use 2 which would then up your voltage from 1 1/2 volts to 12 volts. Well this certainly lowered noise and increased headroom. I even be used a pair of 9 volt batteries on a couple of my units since they were more readily available batteries.

What was most peculiar was that the $30 microphone sounded a lot like the $375 microphone. Why? I had to get to the bottom of this. After numerous phone calls I finally found out that Crown imported their capsules from the manufacturer in Taiwan that made them to their specifications & tested them all and included plots. Where has Radio Shaft went to the same Taiwanese capsule manufacturer and said "we need a bunch of these", which they got. No testing. No frequency response plots. So it was a crapshoot if you got a good one or a crappy one. Out of 10, I got 8 good ones. I gave the other two, to a friend who loves them. They just didn't live up to the quality level of the others in my book. So says most condenser microphones these days have Taiwanese capsules, even cheap microphones can be good microphones if you want that condenser sound. But I don't always want that condenser sound. The bandwidth limitations of the SM57 can actually be quite beneficial when recording acoustic guitar. To make instruments sit properly in any mix, is more about microphones selection than it is twisting knobs. I switch out microphones before I grab at any knobs. So just because you can hear the difference in a microphone that might appear to be "better" doesn't mean that it is better to use that microphone. It's a microphone with a particular sound used for certain applications. And when you end up with lots of instruments and a thick mix, you'll know why one shouldn't always use the "better"sounding microphones. It puts everything on the same plane and it gets mighty crowded sounding.

So nothing wrong with better sounding Radio Shaft Taiwanese made capsules. They just Outlook is fancy as some of the others. So do you buy a car based on its color? Some folks do. I don't. That's why I have 4 different vehicles ranging in size from 500 pounds to 25,000 pounds. From two wheels to four wheels to six wheels. Whatever's proper for the job. Not because it's bigger or faster or better.

Better get this write
Ms. Remy Ann David