There is no good way to stretch your 64-bit seed value without some secret material. Anything deterministic you do is bound to be vulnerable to enumeration of all 64-bit seed values.
The least wrong option is to use a key generation function designed for passwords, e.g. [scrypt][1].

With secret material, you have more options (but still there is the problem that identical 64-bit seed will generate the same output). The basic idea is to mix $Seed$ with $Secret$ into an expanded $Seed'$ using a random-like function. Any [Key Derivation Function][2] can be used, but the simple $Seed'=SHA_{256}(Secret||Seed)$ will do.


  [1]: http://www.tarsnap.com/scrypt.html
  [2]: http://en.wikipedia.org/wiki/Key_derivation_function