Buying a new iMac

[fooloof]

there are several processing speeds available, ranging from 2.4ghz, 2.66, 2.8, and all the way to 3.06ghz. My plan is to buy the 3.06, mainly because I'm assuming that the higher processing speed I get, the longer my computer will be useful to me as the centerpiece of my studio. I wil only be making music (no video or anything) and I expect that I will be getting a lot of sound modules and soft synths (such as Reason 4, Battery, ) as well as the new Spectrasonics Omnisphere (this alone requires 50GB of free hard drive space, for example). I am leaning toward buying the 24-inch 3.06ghz (maxing out the RAM at 4GB) and staying with the standard 500GB hard drive rather than upgrading. The upgrade options are: 750GB for an additional $100 or a terabyte for an additional $250. I am also going with the lesser graphics package since I won't be doing video or gaming.

My questions are: 1. is the 3.06ghz processor overkill?
2. should the 500GB hard drive be enough?

Also, what's the best way to store files? As I see it, there are two main options:

1: Store both apps and files on the computer hard drive and put the files on an external hard drive, too.
2: Store only the apps on the external hard drive and keep all files on an external hard drive (and have an ADDITIONAL) hard drive as a back up to the first one).

I'm on a budget, so I don't want to get more than I need, but still want something that will be current for the next 4-5 years. Thanks in advance for all input.