.. The motherboard

And only the veins exploded and you are now down there with the motherboard.

It still has a free ISA slot?

ISA slots were used to attach computer components into motherboards.

Old computer components, like museum stuff.

There are old computer components in the museum.

Take the ancient GPU and attach it to the motherboard.

Now you should switch it to use it.

In the throne system. Go up there and change it to use the secondary GPU. Exit the system.