Since you're not connecting to FC SAN I assume you have either local storage or IP Storage.
In this case dedicated 1 Gbit link for IP storage is recommended. If you can't have dedicated then you have to share Gbit link between IP storage and VMs.
100 MBit NICs cand be used only as:
1) Dedicated management interface without VMotion or IP storage
2) Standby failover links for management and VM Network
You definitely require at least 2 NICs running on Gbit
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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert
http://blog.vadmin.ru