£300 is a fair price yes, I think you will fine the ISO handling better on the 450D, bear in mind though that the 450D is not a pro body and is a 12mp on a crop sensor so will have some noise but a lot less than an A200 IMO, I have a friend who uses the SONY and he is always complaining about the bad noise even at ISO 400. The 450D is good to 400 but is a bit noisy at 800 although nothing unusable.