For a system running with two Radeon HD 6790 graphics cards in 2-way CrossFireX mode AMD specifies that the power supply have a minimum of 600 Watts or greater with a combined +12 Volt continuous current rating of 37 Amps or greater and with at least four 6-pin PCI-Express Supplementary Power Connectors.
Your Antec Basiq 500 Watt (BP500U) is unable to meet the minimum requirements. The reason is that the PCI-Express Supplementary Power Connectors cable is all on one +12V rail while the other +12V rail is strictly used to power the motherboard and CPU. That +12V rail, that the PCI-Express Supplementary Power Connectors cable is connected to, is also being used to power all of the peripherals like CD/DVD/BR drive, HDDs, SSDs, CPU cooling fan, case cooling fans, etc. The current limiter on that rail limits it to an absolute maximum of 20 Amps which is insufficient for the two Radeon HD 6790 graphics cards and all of the peripherals. The power supply also doesn't have the requisite four 6-pin PCI-Express Supplementary Power Connectors needed for the two Radeon HD 6790s.
What is your power supply budget amount for this upgrade?
I'm assuming that you won't be overclocking your GTX 560 Ti or running some GPU stress testing utility like FurMark or OCCT GPU stress tester. If you do choose to overclock the GTX 560 Ti then you may trigger the 20 Amp current limiter on that +12V rail.
Combined +12V rails make 432W, So 432 - Whatever my CPU + Motherboard is using = Left over watts for graphic card?
Just a guess there
or I'm completely wrong?
Anything that the CPU and motherboard doesn't use really isn't available for the graphics card and peripherals to use in the case of the Antec Basiq 500 Watt (BP500U).
Each +12V rail is absolutely limited to 20 Amps on each rail.
E.g. if the CPU and motherboard are drawing 132 Watts (11 Amps) from its +12 V rail the unused capacity of 108 Watts (9 Amps) is not available for use by the other +12V rail powering the graphics card and peripherals.
Hardware Secrets review of the Antec Basiq 500 Watt (BP500U) shows that you can't draw more than 20 Amps from each rail. Any attempt to draw more than 20 Amps from either of the +12V rails triggered the over current protection circuit.