2 sata to 2 molex to 1 pci -e power cable?
It is quite stable otherwise they couldn't sell the adaptors. I have a similar set-up and it works fine so don't worry about it.
Motherboard doesn't provide that connector for the video card. the power supply does. Take the side panel off and see if it has one. Can always use a 4/6pin adaptor. Most retail cards come with one. Just make sure you have enough watts and +12V amps on the PSU to run the new card. A lot of video card upgrades require an upgrade for the power supply as well.