Trouble running games at lowest resolutions, but i can play normal pc games fine?

I'm new to this whole emulation thing, but i do have, a somewhat, expansive knowledge of computers. what i dont get though is how can i be running L4D2, Portal 2, Starcraft 2, mass effect 2, and a bunch of other games on high quality, but i can only run on the lowest quality possible on pcsx2? I have an AMD Phenom II x6 1045t, a gtx 610 1gb ddr3, and i only have 4 gibs of RAM, but that should be more than enough for ps2 games. can i get some help guys?