Well Pro Logic 2 actually upmixes Stereo to 5.1, True surround sound will be using DTS or DD5.1.
But from what I understand you need to use Optical for surround sound on the PS2, so your gonna need a set like the Logitech Z5400, even if you dont need a set that has Optical inputs, I would suggest you go for something like the Z5400, would be awsome and easy to set up compared to messing with adaptors and stuff, all you need with the Logitech speakers that have Optical inputs is to buy an Optical cable or use either Red or White RCA cable as coax.
Logitech Z5400/Z5450, Z5500, Z680 has PLII support, but the Z5400 which is the lowest of em all is like £135 and they dont make them anymore.