Well, there is a bit more to say. First. the X uses the 3630, _not_ the 3640. Yes, the 3640 is listed as the only 1 GHz model, but it's been confirmed a number of times that the X uses a 1 GHz version of the 3630. This, we're told, is good because the 3630 is better at managing memory than the 3640. Also, like this article states, the X uses LPDDR2 ram and not LPDDR1 like all previous phones (the iPhone 4 _might_ have LPDDR2). LPDDR2 has twice the bandwidth of, and consumes 1/3 less power than LPDDR1. This means better performance at lower energy costs. The same is true when comparing the OMAP to the Snapdragon. The Snapdragon is a 65nm architecture while the OMAP is uses 45nm, which means that the OMAP can achieve at least a similar performance whilst using less energy. It also means that the OMAP has more headroom for overclocking, though that point may be moot on the X anyway.
Lastly, the OMAP is packaged with the PowerVR SGX530 GPU which runs at 430MHz, while the Snapdragon comes with the Adreno 200. I haven't been able to track down much info on the Adreno, but I do know that it sucks. The PoweVR is far superior. I'm uncertain about the difference in power consumption and the performance difference specifics, though.