Load testing is done by automation.
We have to check the response time for clients
for an instance 500kb loaded within 0.2 Secs.

Stress Testing is used to see the Break Point
of an Application or S/W Performance.
It is said to as Performance.

HAhahahah....
Thats the perfect defination mayb but not the answer to the 
guys question.
Its a very wrong conception that Performance testing like 
Load nad Stress and Volume (at times) are always done by 
Automation tools.
    Boss there are many companies who do it manually.
Ok as 4 the test cases, its simple, just write the steps.
e.g. To verify max the no. of attachments which can be 
added.
To verify the maximum no. of users which can be added
To verify the throughput time ( your result will come from 
a stop watch reading)
To verify ma. no of users...
To verify what happens (beyond max. load)
Anything related to performance which is specified in ur 
specs.