Actually according to the rumour they are 3 dual core G6 processors and each can do 2 threads (so 12 in total).
The ATI chip is NOT the one due soon (R420), it's at least a generation ahead (R500).
How would 3 G5 be cheaper than one X86 chip?
Because they won't be high end CPUs, Apple will have those - The G6 is long romoured to appear later this year so Apple may even be onto the G7 by then.
Lower clocked processors cost a lot less money than high clocked processors (especially in the volumes we're talking about) and possibly more importantly run cooler - do you really want a 140 Watt beast in your X-Box? That's what Intel are predicting.
They also can take over tasks like sound and I/O, it's probably cheaper and quicker (to market) to use 3 CPUs than use custom chips.
In any case the PS3 is still likely to stomp all over it...