A computer will WAY outperform a console. And, you can always hook up your computer to your TV and enjoy much higher graphics on the same size screen as your console.
There are three things you might need:Graphics card
as has been said, 6950 and 560ti are both good options. you said you could get a deal on a used 2gb 6950; i would go with that. as was said, test the card by running benchmarks on it to ensure stability, and also to compare the scores (and temps) to scores/temps listed online to ensure full function.CPU cooler (if you want to OC your cpu, which is not necessary, but will probably increase game performance by a few fps)
the hyper 212 plus
recommended earlier is the best budget cooler i know of, but i don't know how much it would run you in india.better PSU (if you are going for 6950 or 560 Ti. 6870 and 560 won't require a new psu)http://extreme.outervision.com/PSUEngine
is a great tool.
when i put in q8200, 2 hdd's, 4 ram sticks, and a 6950 (and i also added 3x 120mm fans and 1 PCIE and 1 PCI card just in case) it gave me a recommended power of 442 watts (446 watts with a 560 Ti).
granted, amperage is the most important thing, not wattage, but in a pinch, your current psu would most likely work. i like to aim about 25% better than the recommended amount, though, and honestly, it would be a good idea to upgrade to a 600 watt or greater psu.