All USB drives should work with the PS3 with one small caveat - they have to be formated FAT32, which requires a little application to do (free) for larger drives. I have a 320 just for this as well, and I transfer stuff to and from my PCs on the same drive. There's a 320G Seagate at Frys for $59 right now. My mom just bought one the other day and she had no issues setting it up - and she's not a hardware kind of person (even though it should be plug and play
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I am not familiar with the airport express, so can't help you on that one.
If the PS3 is connected to a network, he can stream from any PC on the network, it just needs to be enabled to do so between the PC and the PS3.