You should be fine with a 250. Most systems that era use the 5 and 3.3v rails over the 12v and have plenty of room for extra power hungry peripherals. In fact alot of 150 or so watt power supplies powered 486 rigs of that era. EDIT: Also at max a pci slot can draw 25 or 30 watts. So you figure that plus a 30watt cpu at maxish, plus hard drive, optical drives, ram. Lets say 10 watts per drive, two of each. plus another 10 for fans ect. You got 110 watts. Now, most of that is going to be on the old 3.3v and 5v rails, ya got an old psu balanced for a measly 3 or 4 amps on the 12v rail and something like a 486 or early Pentium is going to work great. The important part is quality, system component selection, and rail amperage.

GeForce 4 MX PCI will work with a 486 motherboard.

The FX-5500 probably requires a newer revision of PCI than a 486 board has. And even if you could get it to POST with the card installed, the Windows driver would probably require a Pentium.