Which one should i go for GT520 / hd5670 / gt440 / gts 450
Please give suggestion!!
Give us your total budget. However, all the cards you have mentioned, the best two cards are GTS 450 and then HD 5670. GT520 and GT 440 is a crap card and they are only available in OEM package.
for card 3-5 k, you have 2 choices
HD 5670 512mb/1gb @ 4.5k/5k
HD 6670 @ 5.5k
GTS 450 is better than GTS 250 and costs 6k-6.3k and at least you require FSP saga II 400w @ 1.8k for this to be safe
FSP saga II 350W @ 1.5k would be enough BUT not future upgrades are possible on this PSU
to be safe, go for FSP saga II 500W @ 2k
For your budget of 3-5k HD5670 is the best among the models you've mentioned. GT440 and GT520 are crap cards and GTS250 and GTS450 will probably cost 1-1.5k more than your budget.
my local vendor sadi to me about using the psu of some company called supercomp 500W as it was only worth rs700. is it worth going for it or will it disfunction??
NEVER go for those cheap power supplies if you wish longer life to your PC components...Go with the one Piyush has mentioned and avoid what your vendor is suggesting or you will end up with fried components...
HD 6670 for Rs5.5K seems like a good option IMO....you can get away with an underpowered PSU for now with it...but be sure to upgrade.
GT 520 is a great card for an htpc build but not at all for gaming. Stay away from it.
Gts 450 is a wonderful card for its pricepoint. Its performance is in between a 5770 and 5750. Try to get it within 6k by bargaining, if possible.