it depends, weather you are using the same CHIP manufacturer.
for example, if you are using the nVidia card already, and want to install ATI card. then you have to uninstall the previous drivers and re-install the drivers once you've installed the ATi card. and vice verca.
Ati - nVidia = uninstall previous drivers
nVidia - ATi = uninstall Previous drivers
ATI - Ati = no need, unless u r on old drivers from them
nVidia - nVidia = no need unless u r on old drivers.
* though installing nVidia over ATi wont bother much a nVidia has decent stable drivers,. while installing ATi, you have to completely remove the drivers of the nVIdia, as ATi dont like smelling another card before its arrival. (very unstable drivers)
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"