TouchArcade interviewed Handheld Games Corp's Thomas Fessler, who noted an interesting bit of the company's research: Second-generation iPod touch devices are significantly faster than their predecessors when it comes to 3D rendering.
In fact, it's faster than both iPhone models as well. In its tests, the iPhone 3G ranked second, the original iPhone third, and the first-generation iPod touch lagged behind in fourth. Fessler is paraphrased in the interview, even suggesting he would not recommend anyone interested in gaming to buy a used first-generation iPod touch.
The primary difference is a faster processor: The second-generation iPod touch has a 532 MHz processor. The iPhone, iPhone 3G, and first-generation iPod touch all have 412 MHz processors.
Fessler also speculates in the interview that the GPU has also been improved, but has no evidence to support that claim. These performance differences may also explain why certain apps only run on second-generation iPod touch devices.