The most concrete way to test it is to pull up the window that shows DSP chip usage. When you open one of your bigger sessions how much of your DSP is used? That will tell you a lot. You can also manually count up your voices and make some determinations there. If you are doing pretty big sessions it most likely would be beneficial to add a card. If you want to run more software instruments it's probably a good idea to add a card. Not for the instruments, but for you to move plug-ins off the native processor to create more headroom for your processor to run more instruments.