JRT, do you know the wattage of your power supply? The 5900 was a notorious power hog at the time, but as time has elapsed it may not seem so profligate today. Nevertheless, as your old box, containing an even older MX400, must be of considerable vintage, it may not have enough free electrons to spare for such a card without an upgrade. A quick drift through old Tom's Hardware articles suggests the 5900 might need a 350W supply minimum (no problem to find these days, but you must ensure you get one with all the old style connectors for the old machine!). Also, it usually came in a 2-slot-wide package (to make room for its extra big fan unit as I recall), so it occupied a fair amount of real estate, which may be an issue if you're short on slots.