If you're mixing dynamic and static data on one page, in your case the dynamic data is the logged in user's username, then page caching isn't the right choice. This wouldn't change if you were using file based cache storage instead of memcached.
I suggest trying fragment caching. You can do something like this:
{% load cache %}
{% cache 500 sidebar %}
.. sidebar ..
{% endcache %}
This will cache the contents of the cache tag for 500 seconds with the identifier sidebar.
You can find more information on caching here:
http://docs.djangoproject.com/en/dev/topics/cache/
If this is a page that is going to be hit very often, for example a welcome page, that you feel would benefit from using page caching over fragment caching (for example the only dynamic data is the user name), then there are a few other options.
Say for example you want to have a completely static page except for a login/logout section at the top which displays different links depending on whether or not the user is logged in then you can check for the existence of an authentication cookie when the page is first loaded and conditionally display different data using javascript.