depends on what games you wanna play...
certain games are in reality better on a pc...
as stated above ps3 is good for a media centre aswell if you have bluray discs or are thinking of getting them.... xbox good all rounder games machine and you can stream stuff easily enough via a pc on the same network ..... there is no definative answer as to which one is best ...
i have a good pc and x box .. pc has lasted me for 3 years and is good enough to play most games.. again all depends on what you want to do ....i feel xbox is more what your after however the online part is a good thing.....