Before purchasing this card I made sure to read up on it, almost every forum post confirmed that an e8400 (especially mine, overclocked to 3.6ghz) would not bottleneck this card, but my performance in GTA4 is atrocious and the CPU usage rises to 90-100%. (idles at ~15-20)
1. GTA4 has bad fps unless you have a quad. Otherwise, turn down some of the view distance settings...
2. A stock E8400 is heavily bottlenecked by a 4870x2 at lower resolutions, and bottlenecked to a far lesser extent at higher resolutions(the 4870x2 has the same performance as your card, but requires slightly more cpu power due to its dual-configuration).
An OCed E8400 @ 3.6GHz would still be somewhat to moderately bottlenecked by a 4870x2 or 5870 at lower resolutions, and have a small bottleneck for GPU intensive games at higher resolutions.
Here are some benchmarks for FarCry2:
So you should be barely/slightly bottle-necked in most games, moderately bottlenecked in more CPU intensive games, and heavily bottle-necked in poorly-coded CPU-black hole games such as GTA4.