Raid 5 - 7 drives as good as 8?

Hi,

Trying to get my head round installing some more drives on our Model 170.

Currently has 6 x 4GB drives in RAID 5 with parity spread across 4 drives

so -

              Size    %  
 Unit  Type    (M)  Used 
    1  6607   4194  80.5 
    2  6607   3145  80.3 
    3  6607   3145  80.8 
    4  6607   3145  80.7 
    5  6607   3145  80.3 
    6  6607   4194  80.3 

I have 2 more 4GB drives waiting to go in but I'm unsure as to whether when I 
add the 8th drive will parity automatically get spread across all 8 drives 
rather than 4?

If so that means with 7 drives I get 25.1GB (4 drives at 3145 and 3 drives at 
4194 utilisation)
and with 8 drives I get 25.1GB (8 drives at 3145 utilisation)

Is this correct?

Thanks,

Jennifer.

Confidentiality:  This e-mail and its attachments are intended for the above
named only and may be confidential.  If they have come to you in error you
must take no action based on them, nor must you copy or show them to anyone;
please reply to this e-mail and highlight the error.
Security Warning:  Please note that this e-mail has been created in the
knowledge that Internet e-mail is not a 100% secure communications medium.
We advise that you understand and observe this lack of security when
e-mailing us.
Viruses:  Although we have taken steps to ensure that this e-mail and
attachments are free from any virus, we advise that in keeping with good
computing practice the recipient should ensure they are actually virus free.
Registered Office Address: Markham House, 33-39 Garstang Road, Preston PR1
1JJ
Website address: www.legionsecurity.com