We've got a dual xeon 5520 ESXi machine with a few VMs, none of which are particularly active so we want to rack up some creds in a few boinc projects.

The question is, will we get better performance if we create a single VM with 7 (virtual) CPUs.....or 7 VMs each with a single (virtual) CPU?

It really shouldn't matter. As each 7 VM's running 1 task each will be using the same resources as one VM running 7 tasks. With the newer hardware and software there isn't as large of a penalty for running VM's as there once was. With one VM host there would be less management on your end.

On my dual E5645 I am currently trying to decide if the NUMA configuration effects anything at all.

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On my dual E5645 I am currently trying to decide if the NUMA configuration effects anything at all.

As when the processes are started the memory should be local to the NUMA node of the CPU then it should not make any difference provided the process runs on the same NUMA node all the time. As shown below is none local memory is used things will be slower.