Rick's right about the Getting Started link, but I see one thing you'll need right away: more memory! You're woefully short on it. AE is a hands-down RAM hog, and the more you have, the better.
Personally, I wouldn't even think about running AE with anything less than 8 gigs, and I wouldn't thing about using multiprocessing with anything less than 16 gigs.
I'm going buy a new iMac with these specs:
Mac OS X: Version 10.6.6( Snow Leopard)
Processor Name: Quad-core Intel Core i7
Processor Speed: 2.8GHz
Memory: 8 GB
Chipset Model: AMD Radeon HD 6770M graphics processor with 512MB of GDDR5 memory
Resolution: 1920 x 1080
Is this configuration enough to run After Effects CS5.5..!!
It's definitely a lot better than the system that you aksed about at first. With 8GB of RAM, you'll be able to work pretty smoothly as long as you don't try to run lots of programs at once---and you probably won't get much use out of the Render Multiple Frames Simultaneously multiprocessing feature.
Also, be sure to consider getting at least one more hard drive.
These recommendations are all in the materials that I linked to in my previous post.
Yes, it's enough. However, depending on your intended use, it's just barely enough. I wouldn't advise using multiprocessing.
Consider this: the operating system and the application itself take up about 1 gig. Multiprocessing works best when you can assign 4 gigs to each core. You need to leave about 25% of memory available for other purposes. These are VERY ROUGH FIGURES, but good enough for estimation.
So: 1 gig + 2 gigs = 3 gigs. This leaves 5 gigs for AE processing, image buffering, RAM previews, etc. You're down to one effective core.
If you can double the amount of memory in that machine, you'll be happier. You'll also be happier with a second disk drive.