ASUS P5Q PRO Raid 5 without controller

For your first semi question you referenced would you notice a performance gain?
For RAID 5, the purpose is data loss protection, not HDD performance, hence if you decide after reding this to use RAID 5, you will have a more stable system that is not faster.
If you are looking for a performance increase, you would want to look at RAID 0, which is striping, and does not used parity bits and such. Basicaly it will allocate "stipes" or each information to different drives to speed up the read and then again the write processes for the disks. In this case there is no data protection and you will lose both drives and all data if one fails (note I said both, since commonly you would use only 2 drives for a RAID 0 setup).
And for you additional questions-
1.- The intel ICH10R chip will handle all RAID processes if you use a harware Based RAID setup. If you are simply converting your disks to dynamic disks in Windows and then using the OS to setup the RAID Configuration (called software RAID) your processor will do all the parity bit calcualtions and RAID operations.
2.-So I guess that response answered your second question.
3.-If you are using a software based RAID configuration, then having a quad core would definatley improve your system performance.
Also, as a side note, if you use Windows for configuring a RAID setup, then you must have a seperate disk, not in the RAID, for the OS files and sytem files.
Hope this answers your questions, and i didnt care to look at the post date!
Cheers