CPU-GPU combinations comparison

pure gaming requires more video than processing
the second would be more logic but if you can afford a little more you can buy an HD6870 for much better gaming

Kind of a mixed bag there. You should definitely choose the i5 for your CPU if you looking to game. The 560 in my opinion is a better card then the 6850. So technically it would be great if you could do the i5 with the 560. If not then pick the i5 build with the 6850.

The video cards are pretty much even but I'd get the 2130 CPU. It only has 2 cores but is $60 cheaper than 2400 which has 4 cores but is rarely used in current gaming. It can also Turbo Boost to 3.4GHz which the 2130 can't do but 100MHz is insignificant at those speeds anyway. Overclocking the video card is free, overclocking CPU's can be a bit more expensive sometimes. The 6850 will probably be a bit cheaper than a 560.
With that setup you can get by with a 450W. PSU. Just buy a decent brand which is 80+ or 80+ Bronze certified.

I would suggest getting a P Series or Z68 series MB in case you do decide to o/c sometime. No real reason I can see getting a 2500 rather than a 2500K unless the price on the 2500 drops dramatically.

best combo: core i5 2500k/2400 + gtx 560 ti
if you can't afford it -
core i3 2120 + radeon hd 6870
a bit costlier: core i5 2400 + 6870.
between the two you posted - core i3 2100 + gtx 560 non ti would be more powerful, especially in the gpu bound games like metro or bf3 offline single player.
also keep in mind that ivy bridge might launch in april and more gfx cards from amd 7000 series and nvidia's 600 series will come out this year. those should drive cpu and gfx card prices a bit lower.

Probably should have mentioned, the system is being built in India, so the Ivy Bridge effect(for sandy bridge prices) will come into play way later than it will for you guys.

2500k + 560 Ti is definitely something that overshoots my budget. But since the 560 and the 6870 cost about the same, is there one which is decidedly better than the other amongst these two?

gtx 560 and 6870 trade blows in general gaming performance. but in games with nvidia specific features like physx, fxaa - 560 will perform better than the 6870.
the 6870 earned recommendation for this month's gaming gfx card at its price point:
http://www.tomshardware.com/reviews/gaming-graphics-car...
you can check out the whole article for recommended gfx cards at different price points.

I suggest : the i3 with the 6870. AMD has much more efficient GPUs interms of wattage, thats the biggest reason to go with the Radeon.

the i5 2500 really won't go down in price anymore, not much anyway's. They're releasing Ivy Bridge soon which will replace Sandy Bridge. So the Ivy Bridge CPU that hits the current Sandy Bridges pricepoint, will stay at that price, while Sandy is phased out.