Previous iMac with reduced frequency and low-cost graphic memory (VRAM), laptops with down-clocked GPU, premium price but low-cost parts. And performance lagging far-behind half-cost PC!

This time, it’s the Radeon HD 5670, that ATI specify for GDDR5 with 64GB/s bandwidth. Only GDDR5. Apple put GDDR3 on it to attain 32GB/s, diminishing performance on a 1.5X to 2X factor! That might be a 10$ or 20$ economy for Apple, 1% of the iMac, but a real performance loss for gamers and anyone using GPU, for OpenCL, CUDA, Photoshop CS5 Suite, …

Notice that the ATI Radeon HD 5750 of the Quad-Core 27″ iMac is in fact a HD 5850 Mobility, offering approximately same level of performance, but less bandwidth (as usual).

Steve, sometime you are really cheap!

PS: Preceding iMac 27″ Core i5 Quad-Core had ATI Radeon HD 4850 with GDDR3, but offering 64GB/s bandwidth, having 256bits bus instead the 128bits bus of the HD 5670.

There’s OpenCL Benchmark of Radeon 4870, which show it slower than nVidia 9400M IGP (on Mac Mini, Mac Book Air, …).

Why radeon 4xxx are so slow? They lack “shared memory” that is the equivalent on a graphic card GPU of a processor cache, so every access to memory is 20X to 30X slower than access to cache (or “shared memory” in this case) and it could not compete even if it’s a 400$ graphic card!

If Radeon 4870 could not compete with GeForce 9400M, how a Radeon 4670 will? Having 4X slower memory and 3X less computing power, the Radeon 4670 will be crushed by GeForce 9400M, not talking about GeForce GT120 or even GT130 of last generation iMac!

Even high-end iMac with Radeon 4850 (0.8X slower than 4870 GPU with 2X slower memory) won’t be able to compete with entry-level iMac! It’s sad!