Whats wrong with my PC + boosting the graphic

[HatefulB]

Hi, I remember that when GTA 4 came out it was running pretty good on my PC, on medium.

Now I got an upgraded PC, so it should run just great, but no. The game is just lagging, like i would have some pc from the ice age. My PC is:

AMD Athlon II X4 640 Processor 3GHz

ATI Radeon HD 5800 series, graphic card

4,00GB RAM

win 7

So why is the game running so bad now? Even if i set medium details and have alot of free memory usage in graphic menu (its still green)

I also want to ask, what kind of mods i need to instal to make my game look realistic, like on some of the movies on youtube. I downloaded a 7gb texture pack, but is that all?

At start i want to make my normal game run smooth.

Thanks.