8GB is great. That system has ample power to enable compression
You can't nest vdevs like you suggest. It is just simply that a pool is made up of one or more vdevs. A vdev can be made of one or more disks in some raid type format.
If you wanted an alternative approach, you could mirror...
First, you can definitely take multiple raid groups (vdevs) and combine them into one storage pool.
Another thing that you can do is mix and match drives and sizes even within a vdev. The size of the vdev will be determined by the smallest member disk of course, just like in a regular raid...
Is the eyefinity support on these cards the same as the 58xx cards? As in, would I still need an active dvi to displayport adaptor if I had two of these cards and 3 dvi monitors?
Also, I have a friend with eyefinity, and the colors are off on the monitor he's using the displayport adaptor...