The system assigns resources for adapter option ROMs to allow for capabilities such as remote booting via PXE. As you add more ports, the system can run out of resources and the system might not boot. I suspect that is what you are experiencing.
If you do not need the remote boot capability, you can disable the I/O mapped acess on the Ethernet adapters. Then the adapters will not use up these system resources. If you are using remote boot via PXE or some other remote boot method then disabling the mapping is not an option.
You can use ioutil to disable IO mapped access to the card. Disabling IO access will prevent some pre-boot and DOS driver technologies from working. The utility can be found in the Administrative Tools for Intel® Network Adapters webpack available at http://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=4237.
I hope this helps.