I am wondering what the difference in performance would be between a 8400 G w/ 256 MB vs. the 8400 GS w/ 128MB

I know that the main difference between the cards is the number of rendering pipelines. The 8400 G has 8 and the GS has 16.

would these be about the same?

I cant tell you for sure but
my 6800 has 12pixel and 5vertex pipelines by default and I enabled the rest which makes it 16 / 6 and the performance improvement is big! much bigger then you can achieve with any overclocking

I would not bother that much about the memory my card has 128MB, too and its enough for most games
and since you consider buying a low end card I guess you do not want to play in HD anyway