New build but no display

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hi my new built computer won't boot. Here is the specification:
M3A78 motherboard
GTX 260 video card
2x DDR2 ram
athlon 64 x2 CPU
Rosewill RP 550-2 psu (550watt)
and using my old SATA HD + CD/DVD drive

can some please help? I was thinking bout the psu doesn't have enough A or voltage for this build.

P.S everything was running but no display or no post.
The graphics card requires two PCI-E six pin power connectors. Are they both connected to the power supply? The power supply only comes with one PCI-E connector.
It might be weak power supply with only two rails of 18A each. What kind of DDR2 RAM and which M3A78 motherboard? There are several different ones.
this is my mobo (sorry, i can't link cus i'm new here, need 3 more posts) ASUS M3A78-EM AM2+/AM2 AMD 780G HDMI Micro ATX AMD Motherboard and rams are kingston ValueRAM 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 533 (PC2 4200) Dual Channel Kit Desktop Memory.

P.S on my mobo manual it says make sure the the 20-pin power plug can provide at least 15 A on +12 V. I'm also using 24 pins (the main connector is actually 20 pins but also there are the 4 pins next to it which i connect to the ATX power connector) + 4 pin ATX12V.
The + 4 pin connector from the 20 + 4 pin motherboard connector is not the same as the P4 12V connector that goes to the motherboard. Although there are 4 pins on each, the voltage/ground pin arrangement is different. So you might have used the wrong PSU connector on your motherboard's P4 12V connnection.

BTW, I count 24 pins on the motherboard main connector. Use the full 20 + 4 pins on it.
the psu was ok. Anyway I tested my mobo by removing all gpu, hdd, cd drive, ram etc... except for cpu and its fan. There was no beep sound. So I guess it's a bad motherboard then.
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