Dave, I have a similar setup with a test system - currently at only 2GB of RAM but the rest is almost the same. I'm willing to put up some systems there for stress testing if you want to help me hammer at it. The trouble I have is getting enough people together to do a good stress test. I'd be happy to run a Samba domain, Zimbra mail server (or Sun Java Collaboration Suite), file sharing, and other systems that we can think of. The problem being, again, enough people to really push the envelope. I have a very good edge firewall/UTM system that gives me unlimited VPN connections too.
I'm also willing to stress test Microsoft products in there, but I don't have all the licenses for that many people. I have a 5-user SBS 2003 Standard license I'm not using, but no extra CALs.
As for your SCSI ponderings, I'm really not sure. I don't know how it will perform when you reboot the backup virtual machine. I speculate that it depends on where the SCSI bus hangs, in the guest or the host OS. If its hosed in the host OS, then you're at a bigger loss because it will effect all of your guest systems. For backup, I'm using mostly rsync for data. Since all my production machines are virtual, I run weekly scripts to shut down critical services (like MS Exchange or MS SQL) then backup the virtual machine files to external hard drive. Then I resume the virtual machine. Using LZO compression on the backup keeps the tar.lzo files small without sacrificing much speed (it's almost real-time compression and almost as good as default gzip).
So, rsync with hard links (every hour) and MozyPro (5 times daily) for data file backups. Weekly system images. I feel pretty secure with all the redundancy in my host server (RAID 5 + HotSpare, redundant power supply, 2 different UPS). If I need to bring a system back up from backup, I can copy the virtual machine files to any virtual host, regardless of its hardware configuration, and then restore data backups from either the rsync or MozyPro.