How did you get on with this? We recently had a bunch of problems installing the Proxmox ISO on an R520. In the end we had to install Debian Squeeze and then Proxmox on top (not ideal but it worked).
I have, however, since the install, noted some odd behaviour using VIRTIO drivers on my Ubuntu VM's. The samba fileserver VM I know works, as I have it installed on various other R510/Proxmox setups, appeared to be getting very slow transfer speeds, although running an iperf test showed it as being ok. Any SAMBA/CIFS transfers we were getting ~1.2MB/sec. After many hours of barking up the wrong tree... i.e. thinking it was cables/duplex/switch/ports I decided to try the e1000 drivers on my VM and boom! everything back to normal, getting normal transfer speeds for a gigabit network connection.
Anyone else run into this problem with an R520?
I believe that there are different models of R520 out there with different MOBO's and of course there are many different Dell RAID cards, we had the H710. I will check on the NIC models on our new R520, but the R520 we have that works fine uses the Broadcom 5720's.