The best way to do it is to set a budget on what you want to spend and work backwards.
For processor speed, There’s not going to be a huge difference between 2.8 GHz and 3.2 GHz and you’d probably never notice the difference anyway. As for the number of cores, go with the highest you can afford. I’m not sure how much power you are going to need. For a starter system, the One 2.8Ghz Quad is good (that’s 4 processors right there). The next step up would be the Two 2.8Ghz Quad (8 processors). You could pay the extra for the 3.0Ghz and 3.2, but that’s not worth it.
Final cut pro isn’t written well to fully use the processors, neither is after effects. I use the Nucleo Pro add-on for AFX which maxes the processor usage.
Ram: I wouldn’t go under 4gb. Rule of thumb, I believe, is 2 GB per processor.
Hard drive space is up to you, Graphics, the Radeon 2600 will do the job, unless you want to run 2 30″, then you’ll need a second. Monitor size is your own preference. I’ve found that for dual screen, 23″ was big enough. The 30″ is huge.
Personally, I’m running a 2×2.8GHz Quad (8 processors) with 4gb of Ram, 4x1TB drives and 2 23″ monitors. I wish I had gotten more ram, but the system works great. The only time I wish I had more ram is when I’m doing AFX renders with Nucleo. I don’t get the full potential out the system then.