Again, RAM isn't the issue, as a result of the 256-bit bus avaliable.
Besides, Windows cheats by installing everything to a HD, and then loading into RAM, and usually not deleateing a value after it is used (to save time later). The PS3, on the other hand, will load the majority of data off the disk only as needed, and clean up after itself when data is no longer needed. Essentially, only the background, characters, sound, and the UI will be in RAM at any one point. Cutscene taking place? Close down the UI to free up space (windows would just leave it in RAM to prevent having to re-load it later).
Hence, you can get by with far less resources. This also explains why console releases are far more stable, becuase as a result of not storing everything in RAM at once, an individual bug is easy to track down.
And for the record, the PS2 had a 300 something MHz main CPU. Its all about how you use the hardware, not how good that hardware is.