No such thing as 64 bit After Effects at the moment I'm afraid cs4 or otherwise, it's only just been announced that the next version of AE will be 64 bit see here >
Future versions of After Effects will be 64-bit only - After Effects region of interest
6 gig of ram isn't even 1 gig per core on an i7, as I said Adobe recommend 2 gig per core minimum, 8 cores =16 gig, a lot of people are running AE with 32 gig of ram at the moment as when After effects renders it opens multiple instances of itself to speed up rendering one instance per processor core, each of these instances must be supplied with adequate memory or AE will choke.
The need I have for more processing is due in the most part to the types of compositions I create, mostly 3d and very FX intensive compositions at 1080p, After effects is not a true 3d app and very inefficient at working with it, this is where I've found processing power is needed the most, it's by no means impossible to create good comps using low amounts of processing power (even at HD resolution) that's just not where I'm going with my work.
I'll give you an example. this is one frame from one of my comps, it took 3 mins to render just this frame, it took a similar amount of time to build the frame in the comp, so 3 mins for one frame 29 frames per second means one second of this comp will take 89 mins to render and a similarly long time to preview (ie to process) within AE and I have 8 gig on this machine already at the moment,though it's not the footage size that's a problem, I could render this same shot with no visual effects in a fraction of a second but for the effect to be calculated takes a long long time, don't forget this is a moving image layer that is wrapped" around a shape that is also moving so there's a lot of processing involved. add to that the fact that in amongst this adjustments have to be made, a tweak here and a tweak there can take hours to be calculated and viewed.
This image is also only a single layer with no light sources or camera movements, most comps will have up to around 200 layers throughout the entire comp . . . now do you see what I'm up against, I may be underestimating how good these i7's are but I think your underestimating how much processing After effects can require. Obviously I'm hoping to see some improvement to this comp with a new rig but with the way my machine is at the moment I'm lucky to even be able to open it, it's utterly impossible to work with in any kind of creative capacity.