my tower has a set up to run one monitor but i want to run a second i have a diamond card here in my shop from a old machine im using for spare part's to fix another can i run two cards and a second monitor i have 2 open slots on the mother board free to use the machine i want to use it in is a dell 2400.
Is that a Dell Dimension 2400, if so I think it runs integrated graphics rather than a seperate graphics card! you also need to know what sort of open slots you have there I doubt very much its two pci-e or agp slots. If it is running integrated graphics it may have a AGP slot somewhere as well. I have very little experience of working with integrated graphics... On a more modern machine with graphics cards you can run two cards running seperate monitors if they are both from the same GPU manufacturer I believe. But then most modern graphics cards can drive two monitors anyway.
Your best bet would be to find out what type of graphics upgrade card your system can take and then look for a cheap card to fit that can drive two monitors and disable the integrated graphics altogether.