Perhaps Neverwinter Nights then? Or can't your PC handle it too?
NWN has much higher requirements than PS 
Does it really? I thought it was significantly less, although neither have any trouble running on the sheer awesomeness of an almost new intel mac (few months old, not entirely true: had to turn off some graphics things in Planeshift, so I do get little rgb planes popping up every once in a while, but no other significant problems anywhere). Maybe it was that it was a finished product so much sooner (since it was commercial).
Perhaps Neverwinter Nights then? Or can't your PC handle it too? Then we are left with
Fallout 2
Baldur's Gate 1 & 2
Planescape Torment
Too bad none of these have multiplayer support 
P.S.: At least here we don't see people typing "/shout laaaaaaagggggggggggggggggggggggggggg" 
Which of those are completely free to play?
None, as far as I can tell.