I won't go over the same old ground about the relitive merits of bi-wiring - it's been discussed to death. Suffice it to say that most of the difference (if there is any) comes from removing the metal links on the speakers and doubling the cable cross-sectional area, than the act of biwiring per se.
But I've just noticed you have a 3805 and are not using channels 6 & 7, according to your signature. With the same cables as bi-wiring you can bi-amp. This DOES make a difference especially at higher volume levels. You are not doubling the power to each speaker as the treble unit draws far less power than the mid bass. You ARE separating the drive units from influencing each others output in a way that bi-wiring does not, inspite of the claims. As the levels increase the treble doesn't harden up anything like so soon, which is a lot less fatiguing, never mind the increased sense of control to the overall sound.
Basically you set the rear speaker to outputs to Zone 3 and make sure the speaker levels are matched. It is worth noting, that in the USA where this has been popular, some hum problems have been experienced by some and not by others. The other draw back is that your room equalisation no longer works as Zone 3 is not affected by the 3805s EQ. There are dodges, but I'll only go into it if you're interested. If you're going to buy two runs of cable it's got to be worth a punt. Do a search on
Audioholics.com, there was quite a lengthy article about doing it.
As for which cables, try Van Damme Blue Studio series 2x2.5mm. £1.99/m from Maplins. I've just replaced ALL of my QED Silver Anniversary with it, and it is a much sweeter sounding cable, without any of the QEDs over emphasis of sibilance. I wish I'd found it ages ago - I could have saved myself a small mint. Remember I'm not speculating, this is actual experience from the last week.
FWIW, I've got 2 runs of 6m (I over bought

) on Ebay at the moment. The QED SA sold for enough to mean the VD cost me nothing - result!
Russell