JBOD will be able to use all the space on both disks.....slow.
RAID 0 will be able to use 160gb as you stated, and will be the best performing by far.
My suggestion, forget about RAID anything -especially with the 2 drives you have. Waste of time and performance. Load your OS on the 80 gig drive, and use the 250 for all your programs. That would be the best scenerio performance-wise to use these 2 drives.