They are not terrible cards they are just old. They are 5-6 years old now, so of course they wont keep up with the gpus integrated into a modern i5 or i7.
If you have a Core 2 Duo System chances are your integrated GPU is a GMA X 3500 or a GMA x4000. If that is the case the 8400 would be about 300% faster. Not to mention it would free up the system ram that the integrated graphics is using.
If you have an older AMD athlon x2 or something like that chances are you have an HD3200 or and HD4200. If that is the case the integrated gpu is probably about the same or a little better.