Yes you can swap the existing GPU for a GTX 480, but it will only work in the PCI Express x16 slot (your Fujitsu has one of those).
A 500w PSU will be plenty but get a 600w if you like. Whatever PSU you get, make sure it's got 2 x 6-pin PCIe connector cables coming out of it. They have to be connected to the 2 x 6-pin connectors on the 480, it won't work without those because the PCIe slot alone can't supply sufficient power for it.