PCI question

alright everyone, i've got a pretty stupid question i'm sure. So i've been looking at graphic cards and Motherboards and see the PCI slots and expansions.. i've noticed what seems to be a difference in terms of the interface in a 2.1 and a 2.0 X16 expansion slot. basically what i'm wondering in terms of cards you NEED to make sure that the graphics card and mother board are compatible in terms of this slot?? for example i was looking at
XFX HD-583X-ZNFV Radeon HD 5830 1GB 256-bit DDR5 PCI Express 2.1 x16
video card for a
ASUS M4A79XTD EVO
motherboard but it seems like the motherboard only has 1 2.0 X16 slot or x8/x8 but it looks like the graphics card is a 2.1.. seems obvious enough that these two are not able to work together.  

But seeing how i'm doing a first build i'm trying to understand everything as i go rather then just find parts that work together.. thanks everyone!!