PCI-E 16x (as far as I know) is only used for Gfx cards. Though it would be nice to have a SCSI adapter for a 16x slot. The PCI-E 1x - 4x is mostly used for adapters (USB, firewire, NICs, etc) and I think
4x might be used for some Serial ATA and SCSI adapters.
PCI-E is basically just a MAJOR upgrade for expansion (PCI) slots - Think about it, Gfx cards used to be made for PCI, along with sound cards, usb, firewire, bluetooth and other connection adapters, Serial ATA and SCSI adapters, and some more things. So hopefully in a few years (like maybe 3-5) the cards that were made for PCI will also be made for PCI-Express