If you are using CS3, then I highly suggest you look at some method of transcoding your DLSR footage to a friendlier format.
Cineform is an excellent solution. Avid’s DNXHD codec is also a great choice.
The next question is what are you trying to deliver?
a Blu Ray Disc, a standard DVD, or a web video?
If the answer is anything that is not HD, then you best bet is to bring your HD footage down to SD in the transcode. Your system will likely be much happier during editing.
But if you must deliver HD….then keep your HD as HD (in the friendlier codec) and use PPro’s prefs to scale up the SD source material to match during edit.
Does that help?
Alex