making the pc able to play new games

I want to edit my pc (it is not my main )so it can play new games what should I change
And can I change the case into a bigger one
And will I be able to overclock
Computer :Acer veriton m661
CPU : Intel core 2 Duo E8400 / # of Core = 2 / L2 Cach = 6 MB / Clock Speed= 3 GHz
Memory : 4.00 GB DDR2 (I'll increase it to 8.00 GB)
video card : Intel(R) Q35 Express Chipset Family
Motherboard:eq35m
I can pay up to 800 usd

you can buy a new pc
1.AMD CPU : FX-8 cores Black Edition FX 8120 (AM3+ socket, 125 W power voltage)-140$ plus shipping(you must also buy yourself a cooler that can be 50 to 100$, you should not pay more and another 15$ for the thermal compound)
2.A decent motherboard with AM3+ socket , which would support up to at least 16gb memory dd3, preferably with 4 ram slots(for up to 2 dual channel kits) this can be 100-150$
3. I would recommend you a 8GB DDR3 dual channel memory kit(1600Mhz frequency) from corsair preferably, this would mean around 150 $
IF you can use your power supply and case would be perfect if not get yourself a 800W power supply(50-100 $)
If u calculate the worst case scenario case(meaning that u pay the highest prices ) you would pay by now like 600$, meaning you can have 200$ left to buy a GPU which can be achieved but if u pay 100$ more (meaning 300$ for GPU) you can get a good Graphics Card like Nvidia GTX 660
Now its up to you what will you do, for example if u can use your current power supply and case you dont need to pay extra money for a good Graphics Card and forgot to mention about the HDD you might be able to use your current one but buying a new one (1TB) would cost u 50$
AMD FX 8-Core Black Edition FX-8120 (FD8120FRGUBOX) - See more at: AMD FX 8-Core Black Edition FX-8120 (FD8120FRGUBOX) Details

If you dont want to buy a new pc you should only buy a better Graphics Card but i would highly recommend you to buy a new pc giving the fact that u can pay 800$
So tell me what u want to do and i can give you advice furthermore depending on what you choose.

This is not my main pc but maybe my brother will use it for gaming , I thought of putting a 660 gtx nvidia increase the ram into 8 GB , change the cpu into Intel Core 2 Quad Q9650 and put them in a mid tower ,I want to use the most of this pc and I want to know what people think about it

He is right here , and if ur motherboard supports DDR3 buy 8 GB of 1600Mhz perhaps a dual channel and a GPU this should be enough and u will spend around 400$ if u get a good GPU, yes and change the thermal compound of the CPU buy something good for like 15-20$ and maybe a cooler if this wont suffice, but u must change the thermal compound