Sudden "No Input Signal" on old GPU

[focaner]

I have an old computer (2005) which was working fine until I shelved it for a month to assemble my new one. Today I tried booting the old one up. The lights turn on and the fans (including the GPU fan) start up, but with both monitors I tried it with, they say "No Input Signal", both with HD-15 and DVI-I connectors and multiple cords. Since this can't be the fault of the monitors, I'm assuming that during the month of atrophy something's gone wrong with the video card.

The computer's specs: Athlon XP 2800+, Asus A7V8X-X mobo, GeForce 7800 GS (AGP 8X, DDR). I'm considering replacing the GPU with something like the GeForce 6200 LE in the hopes that that'll solve it.

What do you think? Is there something else I should try with the previously perfectly working GPU to get it running?

 

[focaner]

EDIT: Now irrelevant.

 

[focaner]

Update: Turns out it's neither the monitor nor the video card; the replacement has the same problem. I'm assuming at this point there's a problem with either the power supply or motherboard, but I'm not sure how to test which, or whether it's worth continuing.