+1 for the GA-EP45-DS3. Your x1950 Pro will work in a PCI-E 2.0 slot, no worries.
"16x" PCI-E slots are the longest PCI-E slots on the motherboard, the ones normally used for video cards. You have two of them on the GA-EP45-DS3. If one is empty, the other one runs at full 16x speed. If both are filled (two video cards in Crossfire) each slot gets 8x i.e. half the bandwidth. That is perfectly fine for two x1950Pro cards. It does bottleneck more recent cards like HD 4850 or HD 4870, but not that much (it's mostly visible on 24" monitors or larger).
If you want lots of details:http://www.gigabyte.com.tw/Products/Motherboard/Products_Overview.aspx?ProductID=2843