Practical size limits of a Hashtable and Dictionary in c#

Question

What are the practical limits for the number of items a C# 4 Dictionary or Hashtable can contain and the total number of bytes these structures can reasonable contain. I'll be working with large numbers of objects and want to know when these structures start to experience issues.

For context, I'll be using a 64 bit system with tons of memory. Also, I'll need to find objects using some form or 'key'. Given the performance demands, these objects will need to reside in memory, and many will be long-lived.

Feel free to suggest other approaches/patterns, although I need to avoid using third-party or open-source libraries. For specification reasons, I need to be able to build this using native C# (or C++\CLI).