Not really to defend Playstation 3 here but... 256 MB RAM is ALOT for a video game system (the Wii has less than 100 MB, the XBOX360 has 512 although shared with the graphics card). You forget that it's only ONE, or a very few, applications that is using these 256 Meg (it's really ONE on a Wii). Plus, they optimize it much more than for a PC. Smaller memory also has the benefit in most cases that it's faster and has lower latency.
Also, I think PS3 had 512 MB (256 MB normal RAM, 256 MB separated Video RAM)
Anyway, more RAM doesn't mean faster or better graphics, and not even faster system as a whole. Not necessarily, at least. And then there are a lot more dimensions in a video game than the graphics (gameplay, controls etc. etc.).