would be great if brands differences can be explained. and newegg doesn't work for india so, prefer INR or ebay.in , may be it's too much asking but you can understand how first time gpu buyer scares. one more thing, is there same like 6670 in nvidia ? what you prefer and why ?
I don't know about overclocking because with technical data and practical situations faced by users are very different and close to exact answer.
Sorry you didnt mention anything about India, nor its written on your profile, so i guessed that you would be from US/UK.
What do you mean that if its the same like nvidia? Do you mean whats the offer from nvidia at 6670's price? Probably the GTS450 DDR3 (DDR5 has similar to 6750 performance) if you found it at the same price (i found it only $5 more than 6670).
And dont worry about the brands you mentioned. They are all very good and there is almost no difference between them (HIS, MSI, Saphire, XFX and Asus).
Also as I saw there is no factory o/c 6670 apart from Asus which is only 10mhz so forget about it :P.