Odd problem with a previously-working system

So I've been promising to build my grammy (aww) a computer for the longest time. I've just now gotten around to it, and I'm basically giving her my old system, which will be a huuuuge step up from her 4+ year old Dell Dimension.

Here's the setup:
1GB DDR400 RAM (2x 256MB Corsair ValueSelect, 1x 512MB Ampo)
AMD Athlon 64 single-core CPU, socket 939 (not exactly sure what operating speed)
BIOSTAR 7300GS PCIe x16 GPU
Antec TruePower 430w PSU
BIOSTAR NF4ST-A9 mobo
Lite-On CD/DVD IDE drive
Seagate 7200.10 SATA HDD

Problem:
When I flip the power supply switch, everything fires up. I can't turn the machine on or off by the switch, and all jumpers are properly set. Keep in mind this exact machine worked for about a year. Anywho, all fans spin up. PSU, case fan, southbridge, video card fan, CPU fan, hard drive spins up, DVD drive flashes green like usual, keyboard gets power, even the light on the front of the case flickers. But I get no video, at all. The monitor keeps the amber light, and nothing changes. However, when I undo the monitor cable, it turns green and gives me the 'no signal' message. And, by the way, after the drive and case flicker, they both stop and won't respond.

What I've already done:
- Re-set the CPU and CPU fan
- Tried a brand new Thermaltake PSU
- Tried with no hard drive
- Tried with no RAM, all RAM, only AMPO, only Corsair
- Tried the video card in a different machine, which worked fine
- Taken out the CMOs battery for about five minutes, re-set BIOs with jumpers
- Removed the southbridge fan and re-set it

So, any help, dudes? If I have to buy a new motherboard or CPU, and that's what it's looking like, that's cool, but if I could fix it, I'd much rather do it. I appreciate it, guys.

No luck with that. I'm assuming it's the motherboard?

Alright, I appreciate it. Thanks.