"- 64mb of the 256mb of available XDR memory off the Cell CPU
- 32mb of the 256mb of available GDDR3 memory off the RSX chip
- 1 SPE of 7 constantly reserved
- 1 SPE of 7 able to be "taken" by the OS at a moments notice (games have to give it up if requested)"
"In the case of the PS3 this equates to 12.5% of the available Cores on the CPU always reserved, an additional 12.5% sometimes taken by the OS, 12.5% of the available RSX memory and 25% of XDR Cell memory. Balancing these out, one could argue that Sony has removed up to 25% of the available CPU power and 18.75% of RAM for these features as well as others that are not mentioned here or will be added in future updates to the PS3 Operation System."
Ars technicai posted this info a while back, i can probably find the original thread as well. i dont know if it was ever confirmed but there hasnt been any other changed reports since then.
Twisted, i remember reading the same article about the 360 a while back as well. If i remember correctly, the dashboard/OS/etc uses only 2%-3% of cores 2 and 3 while core 1 is entirely left for gaming applications. i dont remember how much of a memory footprint is used on the 360 but i do remember it being smaller then the ps3's
as far as M_heal's original comment about the memory usage not being the same by a long long way, i think its safe to assume which takes up less memory with all the dev comments about there being less memory on the ps3.