BETTER PERF/PRICE : DDR5 or DDR3

Sorry i cant help with pricing.
What i can do is to tell you that a GDDR5 card is better than a GDDR3 card and then I can link you to a review for performance in Crysis so you can see the difference in the cards.
http://www.techpowerup.com/reviews/Powercolor/HD_6670/1...

Also you should know that the 6670 is basically just a re badged 5670 in fact all the cards you listed are the same. Just get the GDDR5 version of the card in question.
There is another difference in the video side of things with an upgrade to the HTPC side of things. Nothing that makes a difference in gaming.

Mactronix  

HIS Radeon HD6670 Fan 1GB GDDR5 ATI PCI E Graphic Card (H667F1G) @ INR 6085
http://www.theitdepot.com/details-HIS+Radeon+HD6670+Fan...
msi 6670 1gb gddr5 @inr 5955
http://www.theitdepot.com/details-Msi+Radeon+HD6670+1GB...
power color 5750 1 gb @inr 6315
http://www.theitdepot.com/details-Power+Color+Radeon+HD...
his 6750 1 gb @inr 6775
http://www.theitdepot.com/details-HIS+Radeon+HD6750+Fan...
imo msi and his are better brands compared to powercolor. 5750/6750 is more powerful than 5670/6670 for gaming and will get more fps in crysis.
gddr5 is better than ddr3. there is no ddr5 ram or vram.
crysis 2 performance analysis:
http://www.tomshardware.com/reviews/crysis-2-performanc...

which will be better in gaming at 1366x768?

a. 5670 GDDR5 1GB or 6670 DDR3 1GB
b. 5750 GDDR5 1GB or 6750 DDR3 1GB

Answer both a and b.

I wil use the rest as :

G850 @ 2.9GHz,
4GB DDR3 @ 1333MHz,
GA-H61-D2-B3,
Corsair CX 430W,
1 TB HDD @ 7200RPM,
1 DVD/CD Combo Drive,
Compac Case *#???#* <previously used> - equipped with 3 fans for better airflow.


will definetly try to stretch my budget(under 5k = option-a) to 6k for option-b.

looks like an exam question lol.
between a & b,
a. 5670 GDDR5 1GB
b. 5750 GDDR5 1GB
b will perform better than a.
this will outperform all of the above, but seems to be out of your price range.