well, Ill share the little knowledge I have:
You will be needing a RAID controller add on card or RAID support in your motherboard.
How the space in the hard disk is used depends on what RAID you connect it 0 or 1 or 2 .
For example if you want increase in access speed you will be connecting them in RAID 0 here the available space will the minimum of the 2 hdd. In your case 40 GB. This is how RAID 0 works, 1 mb of a file for example is broken into 2 chunks of 500kb and 500kb and each is written in one one hdd, so the speed improves theortically by a factor of 2. This is about RAID 0.
Each RAID (0,1,2,3,4,5,) have different type of configurations and size of the data written(blocks).
Basically RAID is used to improve reliablity and speed.
This is just a outline of it.
Coming to your specific question, I think you will most probably want to connect them in RAID to have an improved performance. then you have to connect them in RAID0.
Please the following link for intel 955 mobo, but it will be similar with other mobos also(I have not tried them but the overall steps are similar).
*support.intel.com/support/motherboards/desktop/d955xbk/sb/CS-020750.htm
