You can never have too much compute power, but the computers you indicate is a good work horse for ATK. If you can afford 12 or 16 GB it's even better; 8 GB is ok; I would not go lower. Considering the price, I would choose a "standard" setup over a specialized one. Xeon processors are great, but i7 is really fast too. Xeon machines tend to make more noise, that can be a factor to consider as well.
At the current moment ATK is not designed to take so much advantage of multi-core CPUs, but this is a feature we are planning to introduce in the hopefully not too far future. Higher CPU frequency certainly improves the performance of the code.
The performance of ATK is a bit better on Linux than on Windows, mostly because of less OS overhead (and no interference from virus scanners etc
). Usually the Linux itself uses less memory than Windows 7, leaving a bit more resources for the applications. The advantage of running on Windows is that it's easier to find good drivers for the 3D graphics.