Is this PSU ok for a micro ATX motherboard

depends on what else you are using in your computer...but it's probably fine

All the motherboard requires from the PSU is the 24 pin power connector and the 4 pin CPU power plug.. Every PSU will have that and so does the Corsair CX 400.. The concern should be over the components that you connect on the motherboard (like video card, processor) and their respective power draws.. For example, if you intent on using a powerful video card like the GTX 480, you would require a higher wattage rating PSU..

There's a radeon hd 5770 and a i3 - 530 in it

You will have no problems running them with the CX 400..

ATI suggests using at least a 450W PSU, but i think the CX 400 is fine

http://www.amd.com/us/products/desktop/graphics/ati-rad...