Windosws Vista 64 or Windows xp 64 ? to a monster computer.

processor http://www.komplett.co.uk/k/ki.aspx?sku=347032

mainboard http://www.komplett.co.uk/k/ki.aspx?sku=343386

Then 8gb ram 1600mhz and a Graphic card http://www.komplett.co.uk/k/ki.aspx?sku=329757

then 6 150gb sata hard drives 10.000rpm raid 0 all then two 22"monitors

First question How many wow chars can i box with this setup ?? Wihout problems lagg or somthing

Second question Windosws Vista 64 or Windows xp 64? I think xp is best for vista take more ram and take mutch of the prossesor but i dunno

Sry for my bad english im from sweden i hope i get answers about this : )