It's important to understand how etherchannel uses the links ie.
backup server = s1
s2 and s3 are devices the backup server sends traffic to.
If the backup server sends 950Mbps to s2 and 950Mbps then providing you are using the right load-balancing method there is a good change 2 of the 4 links will be used, one for each communication.
But if the backup server sends 1.5Gbps to s2 then it will not use 2 links for this. It will only use one link and you will get an oversubscribed link. Etherchannel will not solve this problem for you. This is because etherchannel always sends the same src/dst pair on the same link.
What this src/dst pair are depends on your switch. It can mac-address/IP address/L4 port information.
See this doc for full details of how etherchannel load-balancing works -
http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml
Jon