You need to have a motherboard cable of RAID. Yes, it does double the data rate because it will take the two drives as being one drive. i.e. 2 40GB drives = 1 80GB drive. When you go to write a file to the disk it writes part of it to one drive and part to the other. Hence, twice as fast. Reading from the drive works of course the same way.
As far is it taking twice the space, no. It takes very little extra space (I'd assume some space would go into more complex FAT tables, but nothing noticable) I think when you say "takes 2x the space" you are thinking of another application of RAID technology. Striping and Mirroring are different things.
Mirroring is for the conservative. Basically, you have to 40GB drives lets say. When you save that porn file to one drive, it automatically write the EXACT same file to the other drive. This way each drive has the same data as the other. Given RAID technology, this is not slower, but halves your HD space obviously.