Well, the CPU has some limited Out-of-Order capabilities though not nearly as extensive as X-86 cores. These have to be activated by the programs, which is part of the reason why the PPE in both Xenos and especially the Cell in the PS3 are so hard to program.
Since Xenos is a triple-core, I generally likened it to an Athlon II X3 4xx, which has the same amount of cores. The more modern, much faster architecture should make up for the lack of hyperthreading and some clock speed difference, so an 400e might suffice already.
I think likening it to the older Atom is going a bit too far, at least in performance. The Atom is comparable to early Pentium 4 in performance, both in IPC and clock speed. That's just a bit too slow.
That is very much compiler/developer reliant.
Even an Athlon 2 x3 400e @2.2ghz is likely to beat Xenon.
Remember AMD's Rana has twice the L1 Data+Instruction cache, Out-of-Order Execution, 50% more L2 cache, branch tree predictor, shorter pipeline (12 vs 21 stages), 50% wider core issue width and the list goes on... It's a far more modern and efficient architecture all round. Clockrates are only part of the story.
The 300~ million transistors in the Athlon X3 verses the Xbox 360's 165~ million transistors isn't for show and tell.
Xenon doesn't have an instruction window so large amounts of instruction-level parallelism is pretty much out of the window... But it does tend to rely more heavily on thread-level parallelism. - This is similar in philosophy to the earlier Intel Atom processors. (The more modern Atoms will likely beat the Xenon hands down.)
Decoding of xWMA voices imposes a fixed CPU load of approximately 2 percent per voice on Xbox 360, and a small variable load on Windows (approximately 0.35 percent per voice on a 3GHz Prescott CPU).
So even there a Pentium 4 is showing it's advantages over Xenon.
Another thing to keep in mind is Jaguar which beats the last gen consoles. - Which is an update over Brazos, which in turn was the competitor to the Intel Atom, Rana likely beats that on a per-core basis.