A ND filter will not degrade the image provided that the filter is of a sufficiently good quality. I would suggest that you pay a little more and get a B+W filter. They are really good. Try to stay away from Tiffen filters - I haven't heard many good things about them. As far as B+W ND filters are concerned, they come in the following grades (as Bruce has also mentioned):
[Designation] (Exposure Adjustment) (Filter Factor)
[BW101 (ND .3)] (1 stop) 2x
[BW102 (ND .6)] (2 stops) 4x
[BW103 (ND .9)] (3 stops) 8x
[BW106 (ND1.8)] (6 stops) 64x
[BW110 (ND3.0)] (10 stops) 1000x
[BW113 (ND4.0)] (13 stops) 10000x
[BW120 (ND6.0)] (20 stops) 1000000x
As you may have observed, the ND specification (.3, .6, etc.) is the log of the filter factor.
Hope this helps.