bash script for managing database

Hi folks,

I have two questions here:

(First question) basically I have a bunch of files
1.txt 2.txt 3.txt ... 100.txt and each of them is some results of
calculation done with a parameter "N".

1.txt is done with N=0.99, 2.txt is done with N=0.97, 3.txt is done
with N=0.9 4.txt is done with N=0.89...
As you can see, the filenames are named according to the sort of
parameter N.  The relation is recorded in a file called ./summary:

1    0.99
2    0.97
3    0.9
4    0.89
...
...

Now I am generating a file with N=0.98 in the very same folder.  I
want to rename all the files such that the filename still represents
the order of N, besides, I also want to update ./summary so the new N--
filename relation is updated.

(Namely, I want the new ./summary file to be
1   0.99
2   0.98
3   0.97
4   0.9
5   0.89)

(Second question)
In some other folders I have 1.gif 2.gif 3.gif .... x.gif (x differs from case to case),
and I am about to generate yet another
gif file.  The newly generated gif file is always named "new.gif"

This time I simply want bash to find the biggest number "x" of the
files, and rename my new.gif to $((x+1)).gif.
What shall I do?