If you want to get a good gaming PC which can play games like GTA IV you need to waste around 900$, coz GTA IV needs Geforce 9800 which is also costly and also you need good processor for it.

If you can run GTA IV without any problems then you will be able to run all the games without any problems, coz GTA IV needs v high requirments compared to many games like Fallout 3.
References :

And why the thumbs-down?
I can only assume it would come from someone who knows absolutely nothing about building PCs. I’ve answered plently of questions here where I detailed the parts on good sales that don’t cost much and come together to build a good gaming PC. I’m not going to research all of the prices right now, but you can search right here on Y!A and find plenty of times I’ve already done it.
And I’m not talking about ass like a 9800GT. A GTX 260 Core 216 can be bought for $140 after rebates if you look hard enough.
Go ahead and look.
Now, you’re supposed to be some kind of expert on PCs and you’re suggesting a substandard graphics card? Why would you even do that?
References :
Core i7 Pre-built for $999:
http://www.tigerdirect.com/applications/searchtools/item-Details.asp?EdpNo=5013757&sku=B69-1106&SRCCODE=WEM1994AV&cm_mmc=Email-_-Main-_-WEM1994-_-components
Buy a separate GTX260 Core 216 for $160:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130433
Total: $1159
And that’s pretty much all pre-built.
How much do you think you can save building one yourself?