I decided to go for RAID 0+1, question is how do I do it? I have an Asus P5B board using JMicron® JMB363 PATA and SATA controller it says " Support SATA RAID 0, 1" so I assume I can do 0+1. And I can't find an option on the BIOS that allow me to strip all the HDs into 1, I am going to use 4 x 80Gig HD SATA mode.
 
By the way, what kind of improvement should I be expecting?
 

If you have a standard P5B, you wont be able to create RAID 0+1 with 4 SATA drives.
 
0+1 requires 4 disks, the controller on the P5B supports 1xSATA, 1xESATA and 1xPATA (2 drives) to do it you would need 2 PATA drives and 2 SATA drives (not sure if the ESATA can be ported to internal) The 4 SATA ports that are connected to the Southbridge DO NOT support raid at all on the P5B.
 
If you have a P5B-e or above you can use the ~6 onboard SATA ports that connect to the ICH8R that i cant seem to find any decisive information on wether is supports 0+1 at all.
 
EDIT: After further review in motherboard manuals from ASUS the Intel ICH8R supports 0+1 or RAID10 config, however, this is not present in the vanilla P5B. Please check section 5.4 of your manual for RAID configuration settings.