It depends on the size and distance of the source. generally you'll get good results starting with 8 MP. (but I'd go for 12)
It will be good if your camera has some kind of auto-shutter function, so that you can program a fixed interval.
Getting the lighting right is kinda tricky.
Also you'll need a capable OCR. FineReader and Omnipage 16 have options to perform OCR on digital camera pictures, (correct perspective, distortion, and lighting...)
Keep in mind that using a digital camera, OCR results won't be as acurate.
If you can afford to do destructive scanning, then I'd advise you to get a sheet-fed scanner (check kodak and fujitsu), you'll get much better results.
If you still want to go with your digital camera then check
this thread It has exactly what you need.
Hope this helps