Sig rig won't POST

Could you discribe in a little more detail exactly where you are hanging up like whether you are getting a display or can you access BIOS and any other observations you think might be helpful to us. Fans? Beeps?, etc.

all fans/LEDs turn on, screen doesn't display anything, just keeps switching in and out of the powersaver mode (using an oldish CRT monitor, every 10 seconds or so it acts as if i just turned it on/ just left its powersaving mode)

There are 2 major power connections on your motherboard. One is a 24 pin plug thats hard to miss. The other is a 4/8 pin connector that is easily missed on some motherboards. Make sure they are both connected snugly.

Also make sure your graphics card is seated properly and that the two power connectors on the top card edge are snug.

You might also try removing your CMOS battery out for awhile. This will reset BIOS to conservative settings.

If you can get into BIOS make sure your memory settings match factory timing and volts. Confirm you have RAM in right slots refer to manual for this and reseat RAM modules.

Checked all the connections, everything is plugged in, unfortunately can't check ram timings as I can't get as far as BIOS.

edit: took the battery out (hopefully for long enough), no difference

Try 1 RAM module at a time.

@Mr_Plow I think I've seen you somewhere else before... and I was very careful to keep wires away from fans, and made sure the heatsink was on right. Gunna do albatross's suggestion now, see if that works
Just remembered: If you play maple with that username, then you're in my guild

Oh alomst forgot, disk drive won't open, if that means anything

Also did you reset your BIOS either with a jumper or pulling CMOS battery?

pulling the battery, dunno how to do with a jumper or if I even have everything needed to do that >.>

Edit: Not the ram, unless I have 4 faulty sticks, or my Mobo doesn't like this kind... I really hope that isn't the case

http://www.gigabyte.com.tw/FileList/...a-p55-ud4p.pdf

If your CMOS battery step doesn't do the trick its beginning to sound like something is broken rather than a build issue.

And now I wish I had've checked that list before I ordered the ram... I wonder if there's a list of ram that is known to be incompatible too... they've got OCZ3P1333LV6GK listed, but the serial number on the one I ordered is OCZ3P1333LV4GK.

you have the power connector plugged into the graphics card? somethimes graphics cards can be pushed too far in or on a slight angle I have seen that more than once. What I would do in your situation is to set the motherboard up on the cardboard box that it came in with just the memory and graphics card installed connect all the power connections to the motherboard and graphics card plug the monitor in and short the power button pins you should get a display on the monitor as it tries to POST if you dont then try one ram module at a time to see one is faulty.

Could it be the power supply not working properly?
the LED's on my fans flicker, although fairly rapidly, and as I've been turning the computer on/off occasionally it sounds like the fans are turning on and off fairly fast.

Refurbished PSU did catch my attention. Yes, it is possible PSU is defective.

no beeps, and I'm annoyed because the only PSU's i have are older 20+4 pin ones, so can't actually check if it is the PSU

Another thing to try, On the MB there is the BIOS Flush Pin, sometimes this may come from the factory set to RESET BIOS, and all you do is swap it over.

I had this on a few MB's where there is no life, and everytime (for me at least) it's been the BIOS header set to RESET

The BIOS Jumper is normally next to the battery. It will be a black jumper sat on two pins, there will be 3 pins in total, one pair is for normal operation, the other pair for resetting the BIOS.

On the Board (GIGABYTE GA-P55-UD4P) it's number 20 in the manual _CLR_CMOS and it a little to the left of the lower SATA connections (page 22 - internal connections)

* EDIT .. sorry, my mistake, it has no jumper for normal operation.