Modified warning message and fixed syntax error.

theano/configdefaults.py

@@ -25,7 +25,7 @@ AddConfigVar('floatX',
 )
 
 AddConfigVar('warn_float64',
-             "Warning for float64 output",
+             "Warning, parts of the graph are in float64 which can't be run on the GPU",
             BoolParam(False)
 )
 

theano/tensor/var.py

@@ -575,7 +575,7 @@ class TensorVariable(_tensor_py_operators, Variable):
     """Subclass to add the tensor operators to the basic `Variable` class."""
 
     def __init__(self, type):
-        if (config.warn_float64 and type = float64):
+        if (config.warn_float64 and type == float64):
             print '''Warning, you are instanciating parts of your graph 
                 with the float64 type (possibly by accident).  
                 This means that those parts will not run on the GPU.