Is my 205 watts power supply big enough

Not an easy here, the rest of the load this system is serving also matters. A 100Watt GPU will be pushing the power supply and when you run heavy compute and graphics load the margin of the power supply gets quite close to the edge let alone the heat buildup internally (may want a good thermal monitoring App like https://www.tunabellysoftware.com/tgpro/|TG Pro])

Here's the specs of the GPU's

• NVIDEA - GTX 780M
• AMD - Radeon HD 6770M Your current GPU

Then before you even think of changing the MXM board to something else does the OS support it? Or if not offered reviewing in the case of macOS support review the hardware which is equivalent to yours here EveryMac it may only run in compatibly mode so you aren't getting the full performance of the GPU.