Hello. 
My computer's specs are: 
4 GB RAM 
ATI Radeon 5650 (Mobile) 1GB dedicated graphics card 
i5  M430 (2.27 GHz) processor 
When I play GTA IV, it shows that I am only using up around 300 of my 1080 MB of memory for the game, and yet it is extremely laggy, almost unplayable at times. I would greatly appreciate any tips or solutions to this problem, to reduce lag. 
Thanks. 
PS - I apologize if a similar thread exists on this site - I couldn't find one.