Before anybody can give you a straight answer, you need to tell us what you intend to DO with AE:
Are you a pro doing work for paying customers?
Are you a high school kid making videos for YouTube?
Are you an artist experimenting with multiscreen projection?
What kinds of files will you deliver?
Laptop or desktop?
Mac or Win?
Well i do a variety of things. im completely self educated so ive only recently been getting into professional work. at the rate im going i need a upgade badly. in 1 month ive gone from relatively simple projects to stuff where my gpu had trouble jut rendering the preview. so i guess you can say its a mix between the pro work and the experiments for youtube. im currently working mostly with 29.97fps h.264 quicktime mov.'s seeing as those tend to hold the most quality without being 5gigs a second. up for ideas on that area though. i currently have a desktop as i feel they can hold more power and i can keep upgrading it slowly. currently my budget is limited only by my imagination. as a example (not the best) http://www.youtube.com/watch?v=ELHQexx9GCg that took 40 min to render and about an hour to make. i see alot of flaws in it but i didnt put much work into it so it was more of a learning experience seeing as that was the first real 3d intro ive ever done. most of my work has been in cinema 4d or photoshop. So to sum up the answer i intend to do anything in AE. videos, intros as such movies. i employ the mindset that you should know as much as humanly possible about something in cane it ever comes to need.
Not much point on advising on what you should get when we don't know exactly what you have now, what plug-ins/ effects you regularly use, whether or not you use MP rendering, what your footage types are and so on. You're seeing this too one-sided. For some work disk speed matters a lot. not just raw processing power. Likewise, we could advise you to buy a super expensive card for OpenGL plug-ins, yet it may never come into play if you use MP or otehr effects cause AE to revert to software rendering. Anyway, there's some articles on the help and Mr. Kopriva's blog to get you started. Otehrwise this is an infinitely complex topic and the only true answer is: Get the best you can afford, but get a balanced system. Not much point in buying one super component and then the rest is crap.