According to the manual (specifically, page 96) this is not possible with the Canon EOS 450D. However on page 97 there is this information:
If the [Custom Functions (C.Fn)]’s menu [Auto Lighting Optimizer] (p.156) is set to [0: Enable], a darkly-exposed (set with exposure compensation) image may be displayed brighter.
This might help where your exposure settings result in a too-dark image on the screen, though from the description only in certain conditions (i.e. when using exposure compensation). I don't know whether it will help with overexposed images.