I had a read of the manual and you have a Radeon 7000 AGP graphics card, looking at the picture of the case etc it looks to be a normal size case so the answer is yes the X1550 should fit no problem.
I would to be safe check the power supply is up to it, the X1550 needs a 420 watt power supply or above. If you look inside you case at the power supply there will be a label on it saying wattage. If its below 420 watts you might have to replace it as well. If its a 400 watt you might get away with it but it would be struggling and I personally would not chance it in case it blew and damaged the PC.
You'll have to look and see what it is.