In all guides and postings i've read so far regarding the CPU upgrade inside a Mac Pro 1,1, noone is actually stressing the importance of the stepping to be considered before buying a new processor. For example - the popular X5355 (Quad-Core 2.66 GHz) is available in three flavours: SLAC4, SL9YM and SLAEG. All of those are reportedly running in the MP 1,1. There is an important difference between those, however: Only the SLAEG variant features the latest G0 stepping, which reduces the IDLE power by roughly 50%! The original 5150 Woodcrest processors (DualCore 2,66GHz) have an IDLE power of 24W, while the 5355 Clovertowns (QuadCore 2,66GHz) have an IDLE power of 50W in the older steppings, which is twice the amount and explains, why some people have to adjust fan speed with smcFancontrol to get rid of the additional heat (those are the ones w/o the G0/SLAEG stepping . With stepping G0 (to be found in the SLAEG variants) Intel about halved the power consumption in IDLE state, which brings the x5355 down to 25W, thus being nearly identical to the original 5150's 24W. Of course when fully under load, the Quadcores will sip significantly more power than the Dualcores (after all it's twice the amount of cores at work and that's why the TDP has been nearly doubled from 65W to 120W), but overall heat and power consumption should be significantly lower and closer to the "original" with the SLAEG x5355's. On the other hand, if you don't need more processing power, but want to save on the energy bill (i remember one member here from India being on UPS during regular blackouts), you could try to get the 5150's in G0 stepping ("SLAGA"), because the energy consumption in IDLE state is only 1/3rd (8W compared to the original 24W)! Interestingly enough, the x5365 is said to have the exactly identical IDLE power consumption as the x5355, which makes you wonder whether an x5355 is in fact nothing more than an artificially crippled x5365 (at least with G0 stepping - earlier steppings have a different TDP) that could be overclocked to 3,0GHz w/o any problem... Sources of information: Techreport.com, Wikipedia btw: Did anyone ever try to mix 5150 and 5355 to get a 6-core machine?