see the above sticky for links and videos. "How to mic a choir"
You need an interface and some software to accomplish what you are attempting. The quality of that interface and software will help to get a better sound.
The issue you are describing is one of gain staging. A completely different and difficult can of worms to untangle.
Whether you can survive with just a stereo input of your choir or need all 8 mics at once will determine the size of the interface you need. You will also have the choice of USB/Firewire for the interface which may also be determined by your software choice.
With your budget I would recommend starting in the used category for more options.