That's what I did a few months ago. I upgraded from a 9800GTX+ to a ATI5830... and as a result my g/f upgraded from a 9500GT to a 9800GTX+. Also, her father upgraded from an old FX5200 to a 9500GT (along with a new processor, motherboard, ram, power supply, and case).
So in my case it was a win, win, win situation!
Not that the 5830 was a huge improvement over the 9800GTX+, but it is noticeable.. I'd say about 20-25% or so.