You will want to make sure the video card has 512mb of VRAM or more. 1gb or 2gb is better. 16 gig of RAM will definitely improve your experience over the 4 in your old iMac.
How to tune Photoshop CS6 for peak performance: http://blogs.adobe.com/crawlspace/2012/10/how-to-tune-photoshop-cs6-for-peak-performance.h tml
Jeffrey, can you clarify how a Photoshop user should evaluate GPU RAM when shopping for a computer with integrated graphics? For example, Apple's tech specs for the Mac mini do not list the amount of VRAM for the Intel HD Graphics 4000 used in the mini, presumably because system RAM is used. Adobe's Tested Video Cards for Photoshop CS6 list doesn't seem to be updated for the new 512MB requirement for 3D, and the list of supported graphics cards doesn't make a distinction between GPUs that have less than or more than 512MB.
In an Apple forum I found a post claiming that if a Mac with Intel HD 4000 graphics has less than 8GB RAM the integrated graphics get 384MB of VRAM, and if the Mac has 8GB RAM or more the integrated graphics get 512MB of VRAM. It was a posting from a user, so do you know if that claim is true, and if there is another RAM breakpoint where the integrated graphics could gain use of 1GB or 2GB of VRAM?
And following up to that, is there any way to use a utility or a Terminal command to manually assign system RAM to Intel integrated graphics?
Thanks to Jeffrey and Conrad for this information. I just looked at a site called everymac.com which allows you to compare macs and they say my current iMac has integrated Vram of 256Mb and together with only 4Gb of ram I don't think it is helping. According to this website the new 2.9Ghz iMac has 512Mb Vram (which would be ok) and the 2.6 Ghz Mac Mini has 384 Mb integrated Vram, which can be increased but the maximum Vram is 512Mb.