Well... a Geforce 9400GT definately doesn't require a 980W power supply - it is utterly nonsense if someone told you that. It depends a little bit on the rest of your components but 250W is probably just fine.
If you want some help troubleshooting this problem you definitely need to tell a little bit more because with this amount of info I can't do much. What components does your computer has, what operating system are you running? And what is the message (particular the error code) on the blue screen. If you hit DEL during booting, can you access the BIOS? And have you tried starting in safe mode using F8?
If you can see a blue screen it's not very likely that your graphics card isn't working - otherwise you wouldn't have seen a blue screen at all.