hi, currently i am using com with spec of:
intel E6550
intel motherboard
Sparkle 8800 GTS 320 Mb
PSU = 400 watts
I am just concern whether my PSU of 400 watts supply sufficient power to my graphic card. Thanks for your advise and the link below shows my PSU spec
Depends on what PSU manufacturer is. I would recommend you upgrading your PSU to at least Corsair 520HXEU. Especially if you gonna upgrade your gpu or cpu 
400W does seem a little low for running the GPU beast. Though you haven’t specified what your actually doing, gaming/browsing/video editing. Depending on what you need the power for depends on what PSU you need.
try a 550W or 600W psu for maybe corsair or ocz, 400 I think is a little too low