How can a closed box not outperform a PC of the same specs? That would be downright illogical. Theres no debate about that. Good luck finding PC of 512MB RAM though..
I think you will find that some posters here have argued that the difference is nominal.
Also even the PS3 versus 7800GTX isnt entirely fair..PS3 can use Cell to help..in a way that PC CPU's are not used..also RSX isnt even as powerful as a 7800GTX in some ways..(rops, memory bus etc). But you have to look at things in general terms imo.
I was going to point out that there is give and take, i.e. RSX isn't even up to snuff compared to a 7800GTX (not just rops and memory bus, but frequency, memory footprint, available bandwidth, and so on). And Cell can help... but Cell giveth, and Cell taketh away. The number of multiplatform titles that struggle is an indication that this isn't a sure thing.
But then again that would just go to prove the point that closed boxes get more efficient return.