Game1.cs

  public class Game1 : Game
    {
        private GraphicsDeviceManager _graphics;

        SpriteBatch _spriteBatch;
        private bool _testRenderTarget = false;
        private Texture2D _guiTexture;
        private RenderTarget2D _renderTarget;

        public Game1()
        {
            _graphics = new GraphicsDeviceManager(this)
            {
                PreferredBackBufferHeight = 1024,
                PreferredBackBufferWidth = 768
            };
            _graphics.ApplyChanges();
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            _renderTarget = new RenderTarget2D(GraphicsDevice, 800, 600);
            base.Initialize();
        }

        protected override void LoadContent()
        {
            _spriteBatch = new SpriteBatch(GraphicsDevice);
            _guiTexture = Content.Load<Texture2D>("gui");

        }

        protected override void UnloadContent() { }

        protected override void Update(GameTime gameTime)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();

            _testRenderTarget = Keyboard.GetState().IsKeyDown(Keys.F1);

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);
            _spriteBatch.Begin(SpriteSortMode.Immediate);

            if (_testRenderTarget)
            {
                GraphicsDevice.SetRenderTarget(_renderTarget);
                _spriteBatch.Draw(_guiTexture, new Rectangle(0, 0, 64, 64), new Rectangle(0, 0, 64, 64), Color.White);
                GraphicsDevice.SetRenderTarget(null);
                _spriteBatch.Draw(_renderTarget, Vector2.Zero, Color.White);
            }
            else
            {
                GraphicsDevice.SetRenderTarget(null);
                _spriteBatch.Draw(_guiTexture, new Rectangle(0, 0, 64, 64), new Rectangle(0, 0, 64, 64), Color.White);
            }

            _spriteBatch.End();               

            base.Draw(gameTime);
        }
    }