Refer to the forum Sticky Topic: "300Mbps Connection Requirements! Please Read!"
Specifically, take note of item #2, which requires WPA2/AES encryption.
Note, if you run the network in mixed g&n mode, transfers to all wireless stations will drop to g speeds when a g device is transmitting. The PS3 internal wireless is b/g only. You would need a wireless n bridge (aka "gaming adapter") that would plug into the Ethernet jack of the PS3.
How to get n speeds all the time? Use two routers (one each for n & g) or a simultaneous dual band router.