I cant recall what the low power state of a GTX 280 is . I know they were not as good as current graphics cards which manage to idle at very low power consumption .
Power consumption wise it should work , but have you checked the CX 430 has the power connectors required? Again I cant remember but I think the GTX 280 needs 2 x 8 pin pci-e , but the cx doesnt have them . Maybe a molex to pci-e adapter will fix that . Also maybe not since the molex cant carry the current required for 8 pin
I guess you should give it a go and report back