Even though you upped the power supply wattage you may still have a problem at the +12v rail with the PSU not supplying enough Amps to the card which is why you got a boot but no display.
Although Gigabyte's website only states that a 450w PSU is required to run the card the +12v still has to be taken into consideration. I recommend a PSU @ 600w with a single (or +12v-1 & +12v-2) +12v Rail rated at approximately 45A single or in total, this PSU (OCZ OCZ600MXSP ModXStream Pro.) would run your card, mobo & cpu.
I've been facing a weird problem, my PC Specs are here:
2B PW233 PSU 500W
Intel Core 2 Duo E7500
GIGABYTE Radeon HD 5770 1GB
Biostar G31 M7 TE
Everything was working greatly until I switched on my PC that day, and it wouldn't boot. I thought that my past PSU unit (Digital (R) 450W) was faulty, so I changed it to a newer one (2B PW233 PSU 500W), but that did no good even. When my graphics card is only plugged in the PCIe slot, the computer boots fine and everything works normally, except the display which won't show, because the GPU is disconnected. When I connect the video card with the 6 pin power connector, then push the power button, the PC won't boot at all, and the PC fans rotate a very very very little degree and then stop a.k.a instantly stop, and the green light above the power button flashes green.
\Can somebody shed some light over my situation? I bought this card at a rather high price and I hope it isn't bricked.