In a previous article I guessed that the performance of the Radeon 2600 Pro might not be very good. However, I didn't expect it to be quite as bad as it actually is.
Macworld's benches show that this new GPU is actually slower than the slow GeForce 7300 GT from the previous lower end 24" iMac. In fact, according to Bare Feats' benches, the 2.8 GHz iMac with Radeon 2600 Pro is less than half as fast as the previous top-of-the-line 2.33 GHz model with 7600 GT in some tests, despite the 2.8 GHz model having a faster system bus and a 20% faster CPU.
Some of this may be due to the maturity of the drivers, but it's unlikely that updated drivers will double the performance, which would be needed just to approach parity with the 7600 GT.
This in itself might have been acceptable had Apple chosen to allow us to configure iMacs with a higher end GPU option, but inexplicably, Apple has refused to offer this option.