Also, it would help Serhiy's divide and conquer algorithm if the fast cases included the sides of Pascal's triangle rather than just the top:

   if n < TableSize and k < limits[n]:
       return comb_small(n, k)
   return comb_slow(n, k)

Build the table like this:

    TableSize = 101
    limits = bytearray(TableSize)
    for n in range(0, TableSize):
        for k in range(0, n+1):
            if comb(n, k) != comb_small(n, k):
                break
        else:
            k += 1
        limits[n] = k