They can put all types of fancy stuff in the 450D, but at the end of the day, the sensor ain't gettin' no bigger, and they will probably cram more pixels into it, meaning more noise unless they introduce a better noise reduction system which might be down to the Digic III processor that will go in it, compared to the 350D and 400D Digic II. (350D has less noise than the 400D at higher ISO's 800 and 1600 - as 8Mp vs 10Mp)
I own a 350D and the image quality is great, but remember it is as much down to the lenses you buy as it is the camera.
Personally i'd say for the 400D ASAP, meaning more time of it being the best Canon at that price point.
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever