What Settings are you Running?
GTA IV is processor hog.
You need a Quad Core to run it smoother, But you could try overclocking to 3.5 or 3.6 (FSB 250 x 14 or 14.5) without voltage changes. You need an aftermarket cooler if you will go higher than that.
I got same Processor but lower clocked.
Athlon II X2 245 2.9ghz and HD 6670 GDDR5.
I get 30-32 Fps in benchmarks most of the time (Settings All High- Sliders 25). When i overclocked it to 3.5ghz with stock voltage. I gained 8-9 Fps in benchmarks. Around 25-35 Fps in Real Time Gaming.