I will definitely go for the 17-55mm F/2.8 IS Lens. The 450D is limited to ISO 1600 when compared to 50D's maximum of ISO 12800. However on your 450D, the ISO 1600 images is better than the 50D's ISO 3200. The fast F/2.8 and IS can enable you to get good images ISO 1600, U may not need the 50D high ISO ability at all.
Cheers
Mahathir
Cheers
Mahathir