Well, lets see. Things to check
- 20+4 Pin Power Connected
- 4+4 Pin CPU Power Connected
- 6 Pin PCIe Power to GTX 550Ti connected
- RAM sticks seated properly
OK. So. If you've checked all that, and it still doesn't work. Take all the RAM Sticks out but one, unplug all peripherals leaving only the motherboard and GPU connected to the PSU, and power the thing on.
If it it doesn't work then, you should see if you could swap in a known-working PSU and give it a try. And if that doesn't work, then you're probably at Bad Motherboard/CPU.