I would try to start mining Bitcoin into a mining pool, and I'm looking for a client compatible with Linux (if it has a GUI would be better) and that admit to use my Nvidia GTX 860 graphic card. Any suggestion?
Answer copied from comments:
Because bitcoin mining is now done by dedicated hardware named ASIC miners, the difficulty is now too high for a normal PC CPU + GPU to make it profitable considering power consumption and other things.
See http://en.bitcoin.it/wiki/Non-specialized_hardware_comparison for further info.
Bitcoin's were so profitable at the beginning that specialized hardware have been developed that can mine bitcoins. This hardware is based on an Application Specific Integrated Circuit or ASIC. There are different generations of ASICs that can do more in less time as they become more recent.
It is almost impossible to get any help in setting up a nvidia GPU based bitcoin miner because the folks that develop the software no longer support what they consider a unworthy configuration. You can get low end ASICs below $40.00, perhaps lower. I saw one on Alibaba for $25.00. These low end ASICs are built to plug into a USB hub or port. The simplest way to begin mining bitcoins would be to buy one or more of these USB ASICs and then use software like BFGMINER which can be found for windows, MAC, and Linux.