Best possible specs are as follows:
21.5" 4K iMac:
- 3.3 GHz Core i7 Quad Core
- 16 GB RAM
- Intel HD 6200 Iris Pro Graphics (no dedicated GPU option available on the 4K 21.5" model)
- 2 TB Fusion Drive or 512 GB SSD
- 4.0 GHz Core i7 Quad Core
- 32 GB RAM
- Radeon M395X GPU with 4 GB VRAM
- 3 TB Fusion Drive or 1 TB SSD
What is made clear by this update:
Apple really does not care about graphics performance in anything other than their highest end computers (15" rMBP, 27" 5K iMac and Mac Pro are the only computers available with dedicated GPU's and all of them start at over $2K CDN). The HD 6200 Iris Pro card is fine for any tasks other then gaming, but falls quite short in the gaming department (it is roughly equal to an overclocked R7 250X, or a GTX 560 (non-Ti).
The new 4K 21.5" iMac is a great option for productivity focused users, but Mac gamers will find the performance quite lacking in newer titles. Games will still run, but do not expect to run newer games at anything beyond medium settings at 1080p.
The updated 5K 27" iMac will be slightly faster than last years model, but will still struggle to run any demanding games at full 5K resolution. Benchmarks are not yet available, but expect performance to be about 10-20% faster then the 2014 model. Newer games should run at High-Ultra settings at 1080p.