I'm in the same situation. With the 188.8.131.52. original Steam version of GTA IV, I sometimes get less than 25 fps.
My PC is similar, maybe a little worse: GTX 560 OC + Phenom II x4 980.
But the other version of GTA IV, cracked 184.108.40.206. with L3EVO V1.2 NVIDIA ENB Series, Ultimate Textures 2.0, xliveless, Trafficmodv3 and other mods runs smoother. You should also try the 295.73 WHQL NVIDIA driver, it's better and much stable than the 296.10.