I decided to take some old sunset photos I really like and see what would happen if I created "fake" HDR versions of them using the Photomatix plug-in for Aperture. I opened each photo in Aperture, created two additional versions, and then adjusted one of the versions to have a -2 exposure and the other to have a +2 exposure. After that I used the Photomatix plug-in to create the HDR image.
As you will see, some quality is lost in the HDR versions, but they still have more vibrant colors. There are several reasons for the loss of quality (or "noise"). First, I was shooting with an ISO of 400, which isn't ideal for obtaining the best results. Though you typically want a higher ISO for low-light situations, it can compromise the integrity of the photo when shooting for HDR and create more noise. (So your best bet when shooting for HDR is to set your ISO to 100.) Second, I wasn't shooting with HDR in mind, so the one frame I took was geared to getting the best sky images (not the foreground). Finally, and I've mentioned this before, using one JPEG to create an HDR image isn't ideal.
That said, here are the results: