That is a very good question, and one that is likely very dependent on the exact workload. And to complicate matters, since you are planning to use virtualization on the platform, making the decision can be even more difficult. Since I don't know what applications you will be virtualizing, I will use a couple of benchmark examples.
If your application scales closely to benchmarks that measure integer performance (such as SPEC_int), then the 4 7210's would be better, mainly because the bandwidth is spread across 4 links to the memory controller instead of two. If your application is more transaction-oriented, then you might compare scores using an on-line transaction processing (OLTP) benchmark such as TPCC. Again, the 4 7210's would outperform 2 7330's, and in this example, by a healthy margin.
In performance projections we find that integer and OLTP benchmarks generally tend to favor the platforms that also perform better in a virtualized environment, however, as mentioned above, it is very dependent on the exact application mix. And there are dozens of benchmarks, each with its own measurement metrics.
You can perform similar calculations by yourself by going to benchmarking websites and using actual numbers. Be aware, however, that many of the scores are done with the top-performing parts, so you may have to search for scores for the value parts.
Here’s a good place to start: SPEC Website
The bottom line to this question: 4 Xeon(R) 7210 dual cores will outperform 2 Xeon(R) 7330 quad cores for most enterprise benchmarks.