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All the current gen cards, including your GTS 450, are PCI-E 2.1 card n they are fully backward compatible with PCI-E 1.1. So, get the HD 6770 which is slightly better than gts 450. 2ndly, I don't think there won't be much bottleneck with any cards up to 6770. And regarding pricing, HD 6670 is available around 5.4K in most of the Online shops and around 5K locally. My friend just bought one at 5K + 4% tax today in Kolkata.
Also I think your mobo has PCI-E 2.0 compliant.
Regarding Graphics card prices provided by you, I thik GTS 450 has the best performance/price ratio. But make sure it is the 1 GB GDDR5 version, currently lots of cheap DDR3 versions are available in the market which offers far lower performance than the GDDR5 version. In fact, for all the chosen cards, make sure you're getting the GDDR5 version.
Now tell us what PSU or SMPS you're having? If it is a cheap local PSU from Frontech, Intex or Zebronics then they are not gonna support GTS 450 or other cards. Tell us the exact model number of the PSU.
Also I think your mobo has PCI-E 2.0 compliant.
Regarding Graphics card prices provided by you, I thik GTS 450 has the best performance/price ratio. But make sure it is the 1 GB GDDR5 version, currently lots of cheap DDR3 versions are available in the market which offers far lower performance than the GDDR5 version. In fact, for all the chosen cards, make sure you're getting the GDDR5 version.
Now tell us what PSU or SMPS you're having? If it is a cheap local PSU from Frontech, Intex or Zebronics then they are not gonna support GTS 450 or other cards. Tell us the exact model number of the PSU.