@LMC:
The more I use Aperture, the less I use Photoshop (I have CS3).
Aperture does most of what I need to do with photos, and for the extras, the Aperture plug-in architecture works really well.
Just because I can, I have the Nik package, which includes all the Nik plug-ins for Photoshop, Aperture, and Lightroom all in the one package. And they are very good indeed. I do use most of them, but only when needed. The only one I rarely use is SilverFX, as I don't really "do" black & white stuff.
The Noise Ninja plug-in is outstanding, epecially now that they've upgraded it to 64-bit. It works really well, eg for high-ISO images. Way better than the Aperture noise reduction.
Aperture really does most of what I need - once in a blue moon I'll need to flick out to Photoshop CS3 to do some serious clone-tool removal of something, for example. But unless you really need to do that stuff, you don't really need Photoshop. I haven't got or tried Elements 8 - I gather some people have that set as their external editor.
I should point out that I'm a switcher. I was a PC user, with Lightroom and Photoshop, but actually changed to the Mac world BECAUSE of Aperture. And the more I've used it, the more I have become a Mac convert in total.
I love it