I have a power supply of only 300 watts and no I have no free sata connection cable, so , think it is unthinkable to put gpu cards that require more power for the power supply, so give me a final advice i should invest in a new motherboard or in a more powerful graphics card?
Like I said:
Unfortunately, to mine you'll need a stronger GPU and more important, a good and reliable PSU, this is important for you safety, if your PSU don't have at least this cable to provide energy for the riser, I suspect it's not a PSU to mine.
You already have a motherboard with 2 PCI-e, but you need: strong GPU to mine and a PSU to run this new GPU and the other card 1650
You can buy both: GPU + PSU and have 1 card mining
You can buy: 2 GPUs + PSU, you can sell your 1650 and buy 2 cards to mine
Or if you are serious to mine you can build a rig with Motherboard + PSU + GPUs (you can use your CPU + memory to save some money)
The model of PSU will depends on which GPU you'll buy but generally speaking a 650w PSU of Corsair, EVGA etc can run 2 GPUs