are you sure about that?!
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors
The CX430 has ~350w on the 12V rail, the CPU takes up 100w, everything else 50w max, which leaves you with 150W for the GPU and 50W of wiggle room. The 6870 uses ~130W on load, so yes, a Corsair 400w handles a 6870 fine.
The 5450 asks for a 400W, does it really need it? No. But since over half of the "400W" on the market outputs ~150w stably, then yes.