In order to run one of the best new features in VMware vSphere 4, Fault Tolerance, be sure to check to ensure you meet the specific processor and guest OS requirements:

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<strong>Intel Xeon based on 45nm Core 2 Microarchitecture Category:</strong>
3100 Series
3300 Series
5200 Series (DP)
5400 Series
7400 Series

<strong>Intel Xeon based on Core i7 Microarchitecture Category:</strong>
5500 Series

<strong>AMD 3rd Generation Opteron Category:</strong>
1300 Series
2300 Series (DP)
8300 Series (MP)

Of course certain operating systems have different inherent functionality as well, and for the latest information, be sure check out VMware's KB <a href="http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&amp;cmd=displayKC&amp;externalId=1008027">article</a>.