1) Can my motherboard support SLI? --- No it will not support SLI. It is a Crossfire only board.
2) Can my motherboard support SLI with PCI-E 2.0 cards in PCI-E 1.0 slots? --- PCI-E 2.0 cards are backwards compatible but you'll have a slight performance decrease.
3) With the x16/x4 configuration, if SLI is possible, is it worthwhile? Do they both get scaled down to 4x? --- Your card setup would run at x16,x4, which is not a highly recommended setup for Crossfire or SLI.