XFX 9500GT 1GB for Intel motherboard.

Not sure about FIFA, guess no if its for 2011. If you meant GTA4 then no as well. An Atom CPU is much to weak for that, as is the 9500GT. You really need a Quad core CPU and a 9800GT/4770 to run that game well.

Intel Atom?
What exactly your specs? Is it a laptop?
If it's a laptop then you can't add any graphic card in a laptop...

most Atom motherboard doesn't come with PCI-E or AGP. Meaning, impossible to add graphic card there.

I missed it, Wa1 is probably correct. If he has an Atom its probably a netbook/laptop. Non upgradable, so he's not playing new games with it.

I have intel dual core,2.50GHz ,3GB ram , If i used xfx nvidia 9500GT ,...can i run GTA IV?? Plz help me????

The CPU probably meets the bare minimum requirements for GTA 4, but the 9500GT is still too weak and it is not powerful enough to effectively use 1GB of VRAM.