INSIGHTS:
Photoshop runs very well on the new iMac 2.8 especially if you max out the memory to 4GB (and dedicate at least 2.4G to Photoshop memory caching). The 8-core Mac Pro was only 21-23% faster than the new iMac.
On the other hand, After Effects CS3 gobbles up as many processors and as much memory as you throw at it since it can spawn sub-processes. If you are a serious After Effects user, you'll want a Mac Pro -- preferably an 8-core with 16GB of RAM.
If you are determined to run After Effects (or Compressor) on the new iMac, you'll want to have the maximum 4GB installed.