Just an update: 
While I get an average of 30-45 FPS in GTA IV, I recently bought the D2D EFLC version, and for some reason I get a steady 18-25 FPS. Not too good. That's even when downing the Texture Quality to medium, rather than high. 
Also, I bought the Dirt 2 game, and it's not different from the demo. 
Without VSync, according to FRAPS, I got 59 Minimum, 90 Max, and 73 FPS average. With VSync on, I got 58 min, 62 max, and 59.9 average, a.k.a. perfectly smooth. 
So for anyone looking to upgrade for GTA IV or Dirt 2, a GTX 275 or even GTX 260 will be plenty. That's about equivalent to an ATI 4870, 4890, 5770, or 5830.