Hello again,
Just to inform everyone.
I decided to go with 7850, Sapphire implementation, Dual-X model.
The main reason for which I took this decision is the 7850's big overclocking potential comparing with the 650 Ti Boost.
Other reasons for this decision would be the 256 bit bus, the higher number of ROPs (32 vs 24 as Ti Boost has) and a much higher computational power.
Another arguably reason would be that the next generation consoles (both Xbox 720 and PS4) will have Radeon GPUs inside and that the developers will have in mind that architecture when optimizing the games.
Therefore, on the games ported from consoles to PC, the Radeon GPUs will have a clear advantage over NVidia GPUs.
Ok, all these being said, I would like to thank to all of you for your useful advices.
I'll come back with the first impressions.
Cheers !