Sensors are not necessarily equal. You might have the same number of pixels, but that does not mean that one sensor is not better at capturing color faithfully, dealing with shadows or overblown areas, as well as the 450D I believe is DIGIC 3 where the 5DMKII uses DIGIC 4.
Where another poster mentioned using a 7D, that is a different beast of a camera compared to the 450D
Just because a sensor is the same size, does not mean it is as effective. The footprint of the Nikon D4 and the 5DMKII are effectively both 36x24, but one is 16MP and one is 21MP. I am guessing the Nikon D4 will out perform the 5DMKII even though both have sensor size and the Canon has more pixels.
Likewise, even though 2 sensors may have the same pixel density, does not mean they are equal either. The sensor themselves matter. What I find with my 5DMK II compared to the 550D is I notice more details in the shadows which can be the DR, but I also seem to notice, to me at least, pictures look better and as some say "it comes alive" to me the sensor is better at capturing color and especially neighboring pixels.
450D is not nearly as capable as the 7D, as well as it is one thing to talk about 21MP FF image cropped and then compared to an 18MP APS-C, in which case the 7D image should actually have a higher resolution (not necessarily better, but higher) but we are comparing a 21 MP FF to a 12 MP APS-C... and given the DR, processors, and sensor quality issue, the 5DMKII to me will easily out perform the 450D, even cropped