I, for one, will be stunned if Nintendo goes with a Gekko/G3/PPC750 lineage processor. Why? Sorry to burst your bubble, folks, but the thing is weak as hell!
Let me count the ways:
* No G3 has been clocked faster than 1.1 GHz, ever. The pipeline is simply too short to clock higher.
* The execution resources on the G3 are anemic: 1.5 integer units, 1 floating point unit, 1 load/store unit, and zero vector units. The thing is comparable to a Pentium II in both age and oomph! The couple extra special floating point instructions for the Gekko aside.
* The front side bus (the thing that hooks the CPU to the rest of the system) on G3s (likely also Gekko, but that's information I don't have) is likewise too slow.
* It has zero ability to execute instructions out of order.
* Does not have the hardware necessary to play well with other CPUs - ie no multi CPU or multi-core.
Changing any of these things would require enough of an investment on Nintendo's part that it would be cheaper to just go with one of the other solutions IBM provides. Hell, Nintendo would be better of with a Motorola/Freescale G4 than with a G3, hands down.
We know that they went with IBM, though, so that leaves two realistic options: PPC970/G5 and PPE/Cell. They both have their up sides and down sides. A review:
+ Beefy execution resources: 2 integer, 2 floating point, 2 load/store, and 1 AltiVec
+ Out of order execution (OOOE) - makes it easier to program for, Cell and Xenon, by comparison, will have steep learning curves - also significantly improves performance of code with branches (think AI, game control, etc).
- May only be able to squeeze one into the tiny Rev.
- Can only run one software thread at a time
+ Will be able to run at higher frequency.
+ SMT = can run two threads at once
+ Competitors will be using it, making ports easier
+ Guaranteed dual core at least.
- Anemic execution resources: 1 integer, 1 floating point, 1 load/store, and 1 AltiVec
- Zero OOOE - rumors have developers complaining that the slow down for branchy code is as much as a factor of 10 compared with standard PC cpus of similar speed! Expect it to get better as coders learn some tricks, but the gap will never completely close; the PPE is optimized for multi-thread throughput (which is why each core runs two threads despite its slim execution resources) on the cheap and not single thread speed.
That's it. If Nintendo goes with a PPE don't expect their processor to be any better than the XBox360s. Indeed, going this route Nintendo would likely opt for two cores instead of three, IMHO, with a clock speed of 2.5 to 3 GHz. If Nintendo goes the PPC970 route, expect numbers in line with what's at the top of the post. That is, unless IBM comes up with a miracle - something they haven't been known for these days.
Nintendo could, hypothetically, go asymmetric like Sony (Cell = 1 PPE + 7 SPE; asymmetric because not all cores the same) and have one PPC970 and one PPE in the chip. I don't think it's likely, but I can't think of an explicit reason why it couldn't be done. Just several reasons why it wouldn't be done: harder to program for, IBM would probably ask for a money hat to crowbar the two together, etc. Still in intriguing idea...