You need to figure out how many virtual machines you will be setting up. After than you can decide on how many virtual servers you will need and whether you can use free virtualization software (Vmware server, Xen, etc.) or whether you would be better off buying more advanced software (VMware Infrastructure).
I wouldn't run any other application on a virtual server.
You can have multiple NICs on the host and assign virtual machines to specific NICs if you are worried about bandwidth. I've always used one NIC on the host and it works fine though.
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