FIR Coeff Storage in CycloneIII

I think FvM probably hit it on the head. Coefficients in FIR filters are usually required on every clock cycle, so the number of ram blocks required would be directly related to the number of "real" taps in the design.

Since FIRs can use symmetry, the number of "real" taps or mulitpliers, may not equal the number of filter taps in the system.

IE you can get a 128 tap decimating filter with 4 real taps, but you would require memory elements between taps, and for each coefficient, so it would probably be 4 + 7 ram blocks used. It would not be able to share these ram blocks even though they would be very small in actual number of bits used.

The number of ram blocks actually used will vary significantly depending on your setup of the filter.