^+1
The 5450 is so far down the hierarchy of GPU's it is still a wonder it is even on there. I almost gave up on it. I will link you to it just so you can see for your self plus here is the min and recommended system requirements.
I hope this helps. You will see if you check out where your card is compaired to the 3870 which is a lot better card then yours. Good luck to you.
GTA 4 Minimum System Requirements
•Processor: Intel Core 2 Duo 1.8Ghz or equivalent AMD Athlon X2 64 2.4Ghz
•Memory/RAM: 1GB for XP and 1.5GB for Vista
•Video Card: 256MB NVIDIA 7900 or 256MB ATI X1900
•Hard Disk Space: 16GB free
•Operating System: Windows XP SP3 or Windows Vista SP1
GTA 4 Recommended System Requirements
•Processor: Intel Core 2 Quad 2.4GHz or equivalent AMD Phenom X3 2.1GHz
•Memory/RAM: 2.5 GB for XP or Vista
•Video Card: 512MB NVIDIA 8600 or 512MB ATI 3870
•Hard Disk Space: 18GB free
•Operating System: Windows XP SP3 or Windows Vista SP1
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html