Please look at my earlier post, it all depends on your OS, NIC hardware 
and ethernet switch, you will not gain 100% gain, however, with carefull 
planing & configuration changes, you can atleast gain some benefit + 
failover at the lest.
This is a OS & Drivers funtionality, i use Brodcomm control sute and 
Extream Network BD switch to configure two GB links to my backup server, 
it has been working well for me.

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