8 cores is fine - but 8 GB of RAM is NOWHERE NEAR ENOUGH.
For an 8 core machine you want
24 GB MINIMUM. (a good rule of thumb is 4 GB PER CORE minimum). However, the Westmeres also have the advantage of "virtual cores" (2 threads per core) - depending on software, you may then actually want 48 GB.
But 24GB is a minimum for 8 logical cores in any case, assuming you want to use multiprocessing to good advantage when rendering.
AE and Premiere share libraries, so you can run both without a "substantial" increase in your RAM footprint.
Video Card:
Regarding the suggestion of the nVidia card: At the moment only Premiere takes advantage of CUDA, After Effects does not - YET - However in CS6, AE will be CUDA accelerated.
The CUDA enabled nVidia Quadro 4800 card is VERY expensive relative to the ATI 5770 -
system RAM is more important. Adding a $1500 nVidia card and then NOT buying a few hundred dollars worth of RAM would make no sense. Buy the RAM first,
If ou are using Premeire for EDITING, then you would probably prefer to have a proper SDI/HDMI card such as the BlackMagic Decklink.
the entry level SDI decklink is only $300, and DOES offer real-time effects for Premeire, etc. and support of the Mercury Playback Engine
http://www.blackmagic-design.com/products/decklink/
