Problem With FILENAME As @variable

Hi.

I am trying to create a database based on variables from xp_regread.
Somehow I can´t pass @dBDir as FILENAME.

I get the following error-message :
Server: Msg 170, Level 15, State 1, Line 24
Line 24: Incorrect syntax near '@dBDir'.
What am I doing wrong ?
I think the problem is somehow related to a datatype-problem or
it is simply not possible to pass a variable to a "create
database"-statement.

I have checked the xp_regread-output...naturally :) ...and it is correct.
Hope someone can help :)

regards
Michael

----------------------------------------
Source-code is as follows:

USE master
GO

DECLARE @dBDir nvarchar(128)
DECLARE @dBlog nvarchar(128)

EXECUTE xp_regread 'HKEY_LOCAL_MACHINE','SOFTWARESchool-ProjectIRPF
Database','dBDir',@param = @dBDir OUTPUT

SET @dBDir =@dBDir+'IRPF.mdf'
SET @dBlog =@dBDir+'IRPF.ldf'

CREATE DATABASE IRPF
ON PRIMARY
( NAME = IRPF_db,
FILENAME = @dBDir)
LOG ON
( NAME = 'IRPF_log',
FILENAME = @dBlog)
GO