I've got a 2008 x64 running without problem...
neptune2:/etc/pve# pveversion -v
pve-manager: 1.5-8 (pve-manager/1.5/4674)
running kernel: 2.6.24-9-pve
pve-kernel-2.6.24-9-pve: 2.6.24-18
pve-kernel-2.6.24-8-pve: 2.6.24-16
qemu-server: 1.1-11
pve-firmware: 1.0-3
libpve-storage-perl...