As for the Cell PROC being more powerfull then the XBOX360 PROC nobody really knows. The Cell PROC IBM and Sony have been showing off is for servers, so in general that proc will be a higher quality than anything that the would put in the PS3. This is because simple economics do you want to give away a highend proc in a game machine or sale it in a server and make your money. The PS3 Cell could be more like the Celeron that the XBOX has now, a dumbed down PROC that can be given away cheaply because it can't be sold in anything else. Intel has even said that the while the CELL PROC is impresive, like the Emotion engine, it is over hyped because by the time that it does make it to production, other companies will have a sutable conterpart for the CELL.
Basically no one knows what will be the final SPECS on the X360 and PS3, but I is reasonable to assume that neither will be as impressive as everyone is speculating. The simple reason is because of heat, these CPUs and GPUs will give off alot of heat and if you add a internal HD to either one you even more. With these units possible being extremely compact there is going to be heat issues.
But that just my opinion I could be wrong often I am