Gta 4 running slow

Hey guys, I've been having this problem for a while now. Why does my GTA 4 run so slow? I've the specs that meets the requirements. It's shown below.

System Information
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Time of this report: 1/26/2013, 22:38:20
Machine name: APPHEXIVE
Operating System: Windows 7 Ultimate 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.120830-0333)
Language: English (Regional Setting: English)
System Manufacturer: Apple Inc.
System Model: iMac11,2
BIOS: Default System BIOS
Processor: Intel(R) Core(TM) i3 CPU 540 @ 3.07GHz (4 CPUs), ~3.1GHz
Memory: 4096MB RAM
Available OS Memory: 4022MB RAM
Page File: 3345MB used, 4696MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
DxDiag Version: 6.01.7601.17514 32bit Unicode

Card name: ATI Radeon HD 4670
Manufacturer: ATI Technologies Inc.
Chip type: ATI display adapter (0x9488)
DAC type: Internal DAC(400MHz)
Device Key: Enum\PCI\VEN_1002&DEV_9488&SUBSYS_00B610...
Display Memory: 2003 MB
Dedicated Memory: 248 MB
Shared Memory: 1754 MB
Current Mode: 1920 x 1080 (32 bit) (59Hz)

In GTA ,the settings can only go up to 241MB. What seems to be the problem? Please help!