can you have 2 pci-video card installed on one computer.
If so, do they add the memory, for example: one is 128 mb and the other is 256 mb is your totall memory now 384 mb????
and what about one pci and one agp card????

Yes you can, but it would only do anything for you if you are running more than one monitor. Typically, with 2 pci cards, it is easier if they are different brands. I am not sure that an agp and a pci would work, but it very well may. Also, the video ram wouldnt be added together, they would remain seperate for each monitor.