2.1 is compatable with 2.0 and 1.0
2.0 is compatable with 2.1 and 1.0
1.0 works on 2.0 and 2.1
Which means they all work with each other (I believe).
x16 will work in x8, x4 and x1
x8 will work in x4 and x1
x4 will work in x1
Not sure if the smaller ones work on the bigger ones...
When you go from 2.0 to 1.0 you lose bandwidth and may bottleneck a video card. When going from x16 to x8 there is a small amount of decrease in bandwidth, only noticeable on really powerful video cards. When going any lower than that, you will bottleneck any mainstream or high end card (Though x4 isn't as noticeable). x1 is practicly useless for any modern video card (As far as using that card beyond normal usage, ie surfing the web).