Buy a wireless router to plug into the modem, not the mac
Hard wire the imac to the router with an ethernet cable, and let the ps3 use the wireless.
In my experience PS3 store downloads and online games, are fine given a half decent signal. The downloads seem to be capped at about 2meg speeds anyway from the store even when i'm wired through to the modem.(im on 20mb)
Depending on how far away and whats between the router and the ps3, your results may indeed vary alot tho.