Nikola Aslanov-1338074 said:
Actually the used power is about the same, but 6670 uses a bit less (http://www.tomshardware.co.uk/radeon-r7-240-and-250,review-32874-9.html).
It is possible that the PSU will not be enough for extended GPU load. Restarts are possible.
I agree that 7730 is much better than 6670, it is worth the price difference.
Thanks for the quick solution. I have ordered 7730.
Update - Cancel the order for 7730 and finally opted for Sapphire 7770 ddr5 with Antec VP450P PSU...