ATI Radeon HD 2600 PRO or the NVIDIA GeForce 9400M

I'm not sure about pure graphics performance, but the one with the NVIDIA 9400M is one generation newer. So that iMac has more recent features, such as a Mini DisplayPort instead of the Mini-DVI port and support for up to 8GB of RAM versus 4GB (6GB unofficially). The NVIDIA 9400M supports OpenCL in Snow Leopard (this is the technology that allows the GPU to do non-graphics processing); the Radeon 2600 Pro does not support OpenCL.

On the other hand, the newer NVIDIA 9400M iMac uses integrated graphics. That means it uses system RAM for graphics. Although it has good performance, the older Radeon 2600 PRO iMac has dedicated graphics (and slightly faster clock speed), so its overall performance may be better (not counting the future potential impact from OpenCL and higher RAM limits).

You can compare the specs in more detail by reading the profile pages for the two models here

http://www.everymac.com/systems/apple/imac/index-imac.html

I'd clearly go for the 2600.

I recently made a comparison of a computer with a 2600 XT (that's not even the pro) with a PC with a genuine (not mobile) 9400 GT and the 2600 beat the 9400 hands down. This was a PC performance check (actually Bootcamp Win7), but I guess the differences of hardware will show on any platform.

I guess it's safe to assume that the 9400M will have less performance than a plug in PCI card (even if it's on a PC) and the 2600 PRO will be better than a 2600 XT, so even a weaker 2600 beat a better 9400.

I also remember a few iMac discussions (google for "9400M vs. 2600") where someone said that the 9400 has 16 pipelines while the 2600 has 120 and that the 9400 uses iMac RAM while the 2600 has genuine VRAM.

I'm not an expert in graphics, but unless you are worring about heat and power consumption (the 9400M would be better in those regards) I believe you hardly go wrong with the ATI.

(One more consideration though: If you plan to use Windows with Bootcamp, NVIDIA tends to have better drivers ... I know first hand that the ATI2600 for Windows XP are lousy (they are great with Windows 7 though ... the 2600XT has a Win7 Graphics performance index of 5.8 here on a iMac 20" 2.4Ghz)).

Well, I was coming from a previous iMac with the Nvidia integrated graphics and I am now running with the 21.5" model with the ATI HD and I can notice a difference when playing some titles. GO with the HD 2600 Pro hands down.

Not sure about the older gen model, but the one with the 9400M will probably be back. Just keep your eyes on the +Special Deals+ page for Macs.