The ASUS Eee PC has an PCI Express Mini Card slot . This is a full size slot for a card 30 x 56 mm.This is not easily accessible, being behind a screwed cover on the underside of the unit. One screw is covered with an anti-tamper sticker, to discourage user servicing. According to some reports, later Eee PCs are missing the connector.
Presumably the PCI Express Mini Card slot .could be used to add more flash memory to the system, but most of the cards offered seem to be for WiFi, which is usually built into laptops anyway.
My suggestions to ASUS and other makes of low cost units, would be to provide a standard USB 2 socket in a compartment large enough to hold a flash drive or USB wireless device. This would be cheaper than a PCI slot and a wider range of lower cost devices to plug in are available.