Logic Studio specs according to Apple's website:
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Minimum requirements to install all applications
* Mac computer with an Intel processor
* 1GB of RAM (2GB or more highly recommended)
* Display with 1280-by-800 or higher resolution
* Mac OS X v10.5.7 or later
* QuickTime 7.6 or later
* DVD drive for installation
Available disk space
* 9GB to install all applications and required content
* Additional 38GB to install all optional content (large content packages can be installed on separate disk):
o 10GB for Jam Pack collections
o 16GB for sound effects
o 6GB for surround music beds
o 6GB for other optional content
Additional recommendations
* PCI Express, ExpressCard/34, USB, or FireWire-based audio interface
* At least 2GB of RAM for large EXS instruments
* USB musical keyboard (or suitable MIDI keyboard and interface) for instruments
* Logic Node requires a Mac computer with an Intel Core processor.
* Logic Pro 9.1 and MainStage 2.1 require Mac OS X v10.6.2 or later and an Intel Core 2 Duo processor to run in 64-bit mode. Learn more about running in 64-bit mode
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I am unfamiliar with this product or Mac's in general so I will be basing this post solely on the program's requirements vs my knowledge of computers.
4GB of ram is more that likely all you will need.
* At least 2GB of RAM for large EXS instruments.
Although I don't know what EXS instruments are it's always safe to double the recommended specs.
CPU wise which IMAC are you looking after. The 3.0 GHz Core 2 Duo are fast CPUs and the quad cores are even faster. Depends on how much you want to spend.
Hard drives are getting dirt cheap nowadays so the 1TB is a good option. The HDD's that come with the iMac have a good speed to them.
Graphics card doesn't really matter since you'll be doing audio.