Help in building a new system

I would up the i3-2100 to an i3-2120 for $5 more. You don't need 4 RAM slots or crossfire support on your motherboard with dual channel RAM and a single gpu unless you plan to upgrade in the future. I would go with the HAF 912 instead of your case. Everything else looks fine.

Besides what nacos said, I would also go with this for RAM:

G.Skill Ripjaw 2X4Gb : http://www.newegg.com/Product/Product.aspx?Item=N82E168...

and a Seasonic PSU : http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Otherwise, a fine build

You can have a much better build at that price range which includes an OS. Here'a a build suggestion which goes slightly more, but includes a better CPU, GPU. Have a look:

CPU & Optical Drive: $226- i5-2500K & Asus(Great CPU)

Motherboard & HDD: $135- Biostar TZ77B & Samsung Spinpoint 500GB(Get this before the shell shocker deal of HDD goes away)

GPU: $214- Sapphire Radeon 6950 & AMD EE 4GB(Great Card with decent RAM)

PSU: $60- Corsair CX500(Use promo code to get 10% off, also 500Watt PSU is more than enough for this setup)

Case: $40- Fractal Design Core 100 (or whatever you like)

OS: $69- Windows 7 Home premium SP1 64bit

Total(Excluding Rebates, promos): 744 USD

Rebates: $20

If it is possible for you to stretch your budget, do get these. If not this downgrade the CPU/ODD combo to i5-2400 & Asus worth $196. Save $30

Sorry for posting over here. I didn't wanted to start a new thread for this again.
I also want to build a new decent 'gaming' system but my budget is around 30,000INR ~ 600$(excluding lcd,keyboard,etc). I don't want an i3 rig because it has only 2 cores if i am not wrong. I thought of getting the C2Q instead the q6600 G0 one with 8M cache.
I have shortlisted some of the components
Processor : Core 2 Quad Q6600 G0( Lucky if i find the 6600 now)(I'd surely overclock it to 3.4ghz and max 3.5ghz)
Motherboard : GA-EP45-UD3L ( As i heard its the best for overclocking, but it would have been better if i could get anything with SLI support)
PSU : Cooler Master eXtreme Power Plus 500W SMPS
GPU : help needed, don't know what i should go for( please advice something that won't bottleneck with C2Q)
RAM : Corsair PAIR DDR2 2 X 2GB 800MHZ Desktop Memory( if something better and cheaper please do advice)
Cabinet : Cooler Master Elite 430 Black Mid Tower Cabinet
HDD : Seagate 1TB SATA Desktop 3.5 inch Internal Hardisk
CPU cooler : Don't have any idea with this so please help me out with this

If i have missed out something please let me know. I want a pc for playing 2011-pre 2012 games. Thanks !

Don't go for q6600. It's old and cost same as the latest i3 CPUs
price q6600 (ebay.in)- http://www.ebay.in/itm/Intel-Core-2-Quad-Q6600-Kentsfie...

Instead go for i3-2100 or i5-2400 which are better. Have a look at this comparison between i3,i5 and q6600
http://www.anandtech.com/bench/Product/53?vs=289
http://www.anandtech.com/bench/Product/53?vs=363

The i3 performs better than q6600, due to better technology, 32nm,etc.

I have uploaded two builds for you. They have been sourced from flipkart & techshop. Both sitesare reliable.
The combination of CPU/GPU should allow you to play mos games without hiccup, although overclocking will be limited to GPU only.Have a look:
Build 1:
CPU: i5-2400 -11232/-

Motherboard: Asrock H61 HVS -2830/-

PSU: CM CX430 -2536/-

RAM: Transcend 4GB -1289/-

HDD: Seagate Barracuda 500GB -3985/-

Case: CM Elite 310 -1829/-

GPU: HIS Radeon 6850 -9320/-

Total: 33021/-

Build 2:
CPU: i5-2120 -7060/-

Motherboard: Asrock H61 HVS -2830/-

PSU: CM CX430 -2536/-

RAM: Transcend 4GB -1289/-

HDD: Seagate Barracuda 500GB -3985/-

Case: CM Elite 310 -1829/-

GPU Palit GTX 560- 10417/-

Total: 29946/-

I pretty much liked the 1st build. But is the motherboard good enough for gaming ?
Actually i am getting the C2Q q6600 for like 6000-7000(110$) so i thought of not goin for i5 if the q6600 overclocked performs same like i5.
By the way, what FPS will i get with the 1st build on GTA 4 or crysis 2 ?

The motherboard doesn't factor into gaming or any other performance.

Then what more will i get with the H67 or Z68 ?

With Z68 you get overclocking capability. With H67 you get two more SATA ports or something  

But i think i5-2400 cannot be overclocked. And how would it be if i get a 2500 with a Z68 Extreme3 Gen3 ?

It can be overclocked a couple of hundred MHz. 2500K with Z68 can be overclocked to 4.5 GHz quite reliably.