The RAM limitation is 100% to do with price, and the fact that the RAM used in consoles is significantly faster/moe expensive than what would be found in a sub 500$ PC. XDR in a PS3 and GDDR3 in a Xbox360. 2G of slower ram is not necessarily better than 512M of faster ram. But, it's all relative to the tasks at hand. Gaming benefits allot from fast ram.
In 2006 when the PS3 was released, 512M was not an uncommon amount of RAM in a PC, however forward thinking, and puting 1G of XDR in it (I know 512M XDR, 512M GDDR3) would have increased the price (or increased losses) 100$, and prevented profits for even longer.
However, why we don't see an Asus gaming console, as they could use a TCM chip (http://en.wikipedia.org/wiki/Trusted_Platform_Module) to reduce piracy, is beyond me. Perhaps it would be too easy to reverse engineer, and then make clones, cutting Asus out of the picture.
I've wondered the same thing. This is pretty much what (if the rumours ar true) Sony plans on doing by using a Cell processor again. It will be allot easier for the developers. The same goes for Microsoft (using x86 chips). I suppose Microsoft is doing the 'gaming PC' with the Xbox360 -- x86 CPU based, and as I understand it, Direct X and a Win32 API.