A PSU often come with several connectors.
Usually there are plugs for:
- The motherboard (20 or 24 pins).
- Extra 4 pins plugs to feed extra power to the motherboard (mostly P4 era)
- For general devices (molex)
- For the floppy drive (getting less common these days as floppies are getting ancient).
- Extra pins to feed a graphical card (Either 6 pins or 8 pins) (modern systems).
- Connectors to feed SATA devices.
On your case items 3 and 5 are the most important.
Either your PSU has one or more 6/8 pins connectors to feed a graphical card, or you will need to use something to transform the molex connctors into extra PCI-e power connectors.
Such a device could look as simple as this:
I could use molex splitters, but how do I prevent overdraw on any individual rail?
Go to the webpages of your PSU's manufacturers webpage. Check the PSU's manual.
If it only has a single +12 rail and it outputs enough power to feed the graphical card (once again, use the manual. How much does the card need, how much does the PSU output?) then there is nothing to worry about.
If the PSU has multiple +12 rails then these should be marked on the PSU, or at least mentioned in the manual.
If I split the two molex four pins, could I have them share the eight pin and six pin power connect ions equally?
This depends on your PSU and its internal wiring. The information who got "a 650w PSU" simple is not enough to answer that,
What can I do?
- Buy a new PSU (which seems rather unneeded. 650 Watt is a lot and should be enough to power your PC)
- Convert your two molex connectors to a 6 pin and a 8 pin PCI-e plug.
This will probably work, though I am reluctant to advice this since most molex to 8 pins PCI-e connectors I found draw power from two molex plugs.
- The same as above but also convert the plug going to the CDROM. (when was the last time you used optical media? My last time was when I installed windows XP).
- For completeness sake only: You can buy a graphical card which needs less power.
- If you do not OC, check if your motherboard really needs the 4 pin connector. If it is a P4 then it most likely does need it. If it is a modern system with a 24 pins ATX connector (basically a 20 pins ATX plug with a 4 pins added to its side) then you might not need it and you can use something like the reverse of this:
4 pin to molex to 6 pin PCI-e is rather kludgy though.