I was in your shoes a few weeks ago wondering if CS4 would do well on my dual 2.0 G5. I downloaded the trial version of Photoshop to settle the matter and was quite pleased that there was no performance hit. In fact I would say that it feels a little more responsive. I am on Tiger, and have 6GB of Ram, and the ATI xt 800 (256mg) card.
I haven't tried the other applications of the CS4 suite. I will say that Bridge CS4 is very dissappointing for what I use it for. All I want is a simple viewer and file sorter. This is a dog on my machine - which might be a case of it being older, but still - Bridge CS4 is much slower slower to open folders of images, and the one feature that I find essential has been dropped. In Bridge CS2 you could click/drag files over to any other folder on the left column and drop them in. Sweet - a real time saver. Bridge CS4 will not allow this - unless I am missing something I forgot to set in preferences. For me, Bridge CS4 is unusable and I will continue to use Bridge CS2.
For my initial, and modest uses, I'd say that CS4 is a winner. It has many features that I want to use, and so far I am not noticing any drop in performance. At least give the trial versions a try before you decide.
Mike