DSLR mic for classical concert?

My girlfriend is a classical vocal performer and I tried to record a couple audition pieces for her with my Nikon D5100, but the on board mic practically ruined the audition video. What external mic would be the best for recording vocal performances in a small auditorium?

RØDE microphones rule the market these days.
I think the RØDE Shotgun is an excellent and
affordable microphone for on your Nikon.
To do it perfect you should place an external mic
on a standard near her but that's more difficult and
less affordable, I think.

Have a look for yourself:

http://www.rodemic.com/mics

and (just an example, they're all over the net)

http://www.bhphotovideo.com/...unted.html

We can't hear what you heard that the Nikon D5100 recorded.

The built-in mic is mono - not stereo. Loud audio (even non-amplified vocals, piano, etc.) can be enough to overwhelm the automatic audio gain control resulting in either very muddy audio or static being recorded. The same can happen when an external mic is used - so be sure to use manual audio control and keep the audio meter away from "peaking". And… We don't know where you were in the "small auditorium". If the camera was located at the back of the "venue", then all that audio (i.e., echo, crowd noise) between the camera's mic and the performer are recorded - mics work a LOT better when they are close to the audio source.

Personally, I prefer music recording in stereo, not mono. Either a pair of good condenser mics or a stereo mic is a better option than a mono mic.

Rode, Audio Technica and Shure make good stereo mics - the good ones are not inexpensive. If the mic you select uses XLR connections rather than consumer-grade 1/8" (3.5mm) single stereo jack, then just get a XLR adapter (juicedLink or BeachTek). They have audio gain control knobs so you don't have to mess with the silly menu option audio gain controls in the camera's menu - just move to manual audio and use the XLR adapter audio gain controls.

In any case, being at the back of the room does not help matters. The mic(s) should be to be closer to the subject. The audio level displayed by the camera need to be monitored. Even better, don't record into the dSLR - use an external audio recorder (like a Zoom H2 or H4) and use that audio when editing the video. And be careful with your dSLR. Read the manual: there are video file size limitations, video sequence duration limitations and overheating problems when used as a video capture device. So when the camera shuts down because of these known, published, limitations, at least you'll have the audio. If you do not want to deal with these known limitations, use a camcorder.