Thanks. If I use mainly Photoshop for large document canvas with many layers, is a separate GPU needed or will the onboard graphics card do? Will there be a big difference?
What you want is fast cpu not multi core, a lot of ram 32gb to 64gb for large format and any basic low end gpu will do for gpu related filter, it doesn't really has many gpu accelerated function, a good i7 or i5 igpu will most likely be enough.
For your reference, i've tried 16gb for wall poster of 5m x 8m, is super slow, but its a side project and i rarely do big format. Its more than enough for my usage of normal a4 to a3 presentation.
Adobe bottleneck is adobe itself :/ even with good hardware it cant run fast.
Imo a competent ps system could be a recent i5 like 8600 + 16gb ram. Use oc i7 + 32gb ram if you wants more raw power.