You are on 64 bit, so don't worry about AWE.
By default, SQL server will dynamically allocate memory. This is configurable to a window of dynamic memory (set a min and max) or you can set it to allocate a specific amount of memory.
SQL 2000 did not do an efficient job of dynamically allocating memory, but SQL 2005 does it rather well (MS made big improvements here).
Every system is different. It is best to start with dynamic memory allocation and do some performance testing to see if you need to set limits or specify memory allocation manually.