Will it improve performance to install an older videocard?

"Yes, it should. With a 9500, you most likely won't see major improvements, but you could see some.

Also depends what the older video card is. If it an older nvidia card, the will most likely be ok, but it may complain a bit having say an old ati, matrox, etc. User experience this will vary. As long as the OS supports both cards, I would say you should be ok, but mixed video drivers can sometimes get interesting."