What motherboard are you using? I was using a Asus A8N32-SLI with my 4805SAS, but could never get the system stable.
Supermicro H8DCE it has dual everything (almost). Dual opteron, dual nForce chipsets so it has 2 full 16x PCI-e slots, and 2 8x PCI-e slots (4x lanes connected, 8x physical slot) and 3 good ol' PCI slots. Dual Lan, Dual NVRAID (from 2 chipsets) etc...
I have an Areca ARC-1260 in one 16x PCI-e slot, and the Adaptec card in an 8x slot. Boot takes forever initializing the raid controllers, especially the 4805, but after that the dual, dual-core opterons are blazin and aren't bottlenecked by storage .