We'll know next week, as Aperture just started shipping.
My guess is the latest iMac will NOT be smooth with Aperture, at least if you start stacking several effects, because of the iMac's lower-mid-range GPU and its single CPU. Recommended are dual G5s and a faster GPU.
I hope to see Aperture on display on iMacs in a few weeks though, just so I can see for myself how (un)smooth it is.
P.S. You might want to check out the Applications forum. There is a
thread about Aperture there.