The card should work properly on your current motherboard .
Now for your 2nd question- Whether 400w is nuf or nt? Dat depends upon how many onboard devices are connected(no of hard disks,drives,pci cards etc.)
400w may be ok and i say again may be ok for your card, but it would be much better if you get a Corsair/Coolermaster 500w SMPS. The current SMPS of 400w may not be able to deliever full power to your components and may lead to later problems. So it's best go for new SMPS as well.