The PC version of GTA 4 has CPU power for breakfast - independent on clock 
speed, L2 cache or number of cores. The latter ones are very important, 
because without quad-core power GTA 4 is noticeably less enjoyable. For 
example: A Q6600 is about 52 percent faster than the dual-core E6600 when 
both CPUs are running at 2.4 GHz. If the clock speed is increased, the 
difference gets smaller, but is still huge and a QX6850 is 47 percent faster 
than an E6850. Dual-core processors have a serious disadvantage and only 
with 3.6 GHz an E8500 is able to beat the Q6600, but GTA 4 nevertheless 
suffers from frequent lags on dual-core systems - no matter if an AMD or 
Intel CPU is used. The Phenom X4 9950 easily passes the Q6600 and 
theoretically is on the same level as the Q6700 (not included in the 
benchmark).

http://www.pcgameshardware.com/aid,669595/Reviews/GTA_4_PC_CPU_benchmark_review_with_13_processors/

quad rules!