I fix these all the time at work, ive seen some of these come with an ati graphics chipset instead of the intel. You would have to get a new motherboard with the ati graphics on it.
I believe the nc6400 hase been discontiunued but you should be able to look up parts for it on hp's part surfer.
http://partsurfer.hp.com/cgi-bin/spi/main