mysql 5 vs 5.5 memory usage
I don't know exactly if 5.5 require much memory than 5.0 but look this example : 

In 5.0 MySQL Official Documentation:

innodb_buffer_pool_size : The default value is 8MB.


In 5.5 MySQL Official Documentation:

innodb_buffer_pool_size : The default value is 128MB.


If you paste your both "show global variables" results (5.0 and 5.5) we'll can see exactly what variables have gone up...

Max.