2 hard drives mate, trust me, you don't want to be splitting a HDD into two diffrent partitions, it just gets to messy. x2 500 gig will be more than enough. although it depend on what type of gamer you are...
if you like to have games on your HDD for fun [or to look good] then go and get a 2T HDD for the PS3 and dump everything you have onto it.
320 gig will be plenty for the wii as most games are scrubbed and take up around 1gig max is 3.5
or save your money on a HDD and go and buy a PSJailbreak and downgrade your PS3! haha