Something is wong with my pc..

hello eweryone I have
Processor Intel(R) Pentium(R) 4 CPU 3.00GHz
Manufacturer Intel
Speed 3.0 GHz

Video Card NVIDIA GeForce GT 520
Manufacturer NVIDIA
Chipset GeForce GT 520
Dedicated Memory 1.0 GB
Total Memory 1.7 GB
Memory 2.1 GB

and I think it should handle gta IV without lag at least at medium settings, but I can't even play gta san andreas on max settings without lag. i dont know what i need to do i think if i upgrade my ram to 8 then i will have less lag... but idk.... i have 400w power block

Observe the Task Manager processes to see what might be hogging resources during stress. This is explained more in depth in Troubleshooting Steps for Windows 7 including a demonstration video for how to use Process Explorer.

As you observe this watch the RAM and Processor usage in various stages of stress. This will clue you to whether you need more RAM or if more RAM will not help because the processor is maxing out before the RAM anyway.

If this doesn't help then complete the rest of the Troubleshooting Steps for Windows 7 which uncover most problems and offer useful optimizations that work best for Win7.

I'd also look over and compare your install to the perfect install compiled in Clean Reinstall Windows 7 because you might benefit by investing in the best install possible, and it will be necessary anyway if you want more than 3.5gb RAM to install 64 bit. Once adequate hardware is tested to be sound, then the install is everything.

But what will happen if i upgrade to 8GB?

my friend said that my power block is too weak because its only 400w and he said u need atleast 600w but not too much because you can burn you pc..

There is no too much :) It is possible to burn hardware with a cheap power supply though.

I also explained this in my post above. I told you how to determine with certainty whether your processor can use the extra RAM, and to try to figure out what might be hogging resources.

I explained that it requires a Clean Reinstall Windows 7 if you want more than 3.5gb RAM to be usable, but the reinstall may also be necessary if you're still using an old corrupt Factory Install which is the worst install of Win7 one can have to begin with.

If you need to calculate power needs use this Power Supply Calculator

and wHAT exacly i need to upgrade for the best performance combination?

Since you show no interest in troubleshooting the present problems I would then first recommend doing the clean reinstall to know you have the best possible installed Windows 7.

If it's doesn't resolve the problems then you can deal with upgrading the hardware once you do the comparisons with the CPU to see that it can support more RAM.

If you have any questions about the steps I suggested then you should ask them back and not just ignore them.