If you are into entry level model, then I don't think you are overly concerned about the noise level....
Of course Canon CMOS sensors are one of the better ones in terms of noise level and I agree as I seen pictures taken at ISO1600 by EOS1000D is quite clean that I thought it was ISO400...
Anyway, I was in the same situation as you, comparing between 450D and D90 only 3 months back...
Sony has never came into my mind at all as I "still" considered them as new-comers in this field....
However, a trip to the COMEX last year completely changed my mind and I bought a A300 instead of the other 2...
Of course, I can anytime afford a D300/D700/D3 and 50D/5DmkII at that point when I was deciding....but it still voice down to why you need DSLR in the first place...
I need a small and handy camera to give me better quality pictures over PNS and my wife can also use it to take pictures of our son occassionally, so the SUPER FAST AF in LiveView and how easy to use the camera won us over!!! :thumbsup:
Noise level wise, I dun think it's a big problem as long as you don't print bigger than A3 size even for ISO800 is still quite ok...or do you ever need to print out that big in the first place....
In summary, 450D definitely has better noise control over A350 but not very obvious unless you always look at your pictures at 100% all the time.
Colour rendering I would say Sony has better and more vibrant colours with some tweaking in camera...
Handling wise, I find 450D too small and grip is very uncomfortable when holding for long period...A350 feels more solid and it gives you the $1.3k feeling rather than the $500 feel 450D gives...
LiveView, if you need it as much as I do, A350 is now the king of the hill for AF, no one does it better as this point in time...
BTW, only Canon L lenses are good and they are also very expensive...