No signal on startup

[omerm]

I have an x850xt and whenever i plug it in my agp slot, and power up, the computer boots up normally, but there is no display. Instead I get a no signal message and my monitor stays blank. all the fans work (including the gpu's). Also this happened with my older motherboard, but when i got it fixed the shop said it was a problem with the mobo. So, I replaced it with an ASUS A8V-MX motherboard and an Athlon 3000+ chip. However, using onboard graphics, the display comes up, but when i switch to the x850, it gives the same signal. I have a 400W PSU (I doubt it's quality though), and i've got my gpu connected along with the hard disk (using a split). I also tested it on another mobo of the same model, and using the same PSU, however, it still does not work. The integrated graphics work and my old mx 440 also works. has my gpu fried, or is there a problem with the model of the mobo? I've seen numerous posts of similar problems, but with no solutions. also, it may be of interest that my RAM is shown as 960mb in dxdiag, whereas i have 2 chips of 512 :/. Specs:
ASUS A8V-MX
Athlon 3000+
Ati x850 xt 256mb agp
1 gb ram (2 x 512)
400W PSU

Please help i am desperate =(

 

[mechBgon]

AGP slots have two layers of contacts (see the third and fourth pics on this page), so start by making sure it's really fully seated into both layers. I remember one system where it took a lot of force to get past the first layer.

Your missing RAM is being used by the onboard video chip.

 

[omerm]

Should I disable the onboard graphics or something? Or will they disable automatically once i plug my gpu in? Also, my card is firmly seated in both layers

 

[omerm]

Sorry for a double post, but i noticed that while my mobo has a 24pin socket, my psu has a 20pin connector, and i've connected the motherboard with 4 pins not connected. Could this be related to the problem?

 

[mechBgon]

It could be lack of power output, lack of power delivery due to the missing wires, or poor power regulation. All in all, if I had a nice video card like that, I'd go get me a respectable 24-pin PSU to go with it. Seems justifiable.

 

[omerm]

Thanks I'll try a new PSU and if it doesnt work off to the trashcan with it