Help required

Hello,
This is my first post, and it seems like people here really know what they are talking about, so here goes.
About a month ago, everything was proceeding as normally on my PC, when it just turned itself off, no warnings. I turned it back on again, and it just managed to load Windows and then it turned itself off again, for the final time. I took it apart, reseated everything, unplugged and plugged in everything, various RAM configurations, but still nothing. The only thing that was working was the LAN light LED. I concluded that is must be the PSU that had blown as i was running a 6600GT and 3 hard drives, and it was only 300W. After trying an old 230W PSU, the fans spun up, and the hard drives whirred, but no video output. I didn't think too much about this as I merely thought the PSU didn't have enough guts.
I purchased a new 480W PSU, nice and shiny black with braided cables, and proceeded to install it. Power on. Nothing. Did all the reseating again. Nothing. Decided to try plugging in one molex at a time. All was fine until i plugged in the 6600GT, it just shut down, back to square one.
Then I tried a spare AGP card (which was working in the spare PC I took it from) that didn't require a molex power connection. It worked, sort of. Everything spun up, but no video output, as if I hadn't even connected the cable, and there were also 2 POST beeps. Decided to reseat the only thing I hadn't reseated which was the CPU. After I had done this there were no beeps, everything spun up, but still no display. So I tried an old PCI (not PCI-E) graphics card (which works fine in the parents PC I borrowed it from) and still nothing.
I believe this may be a motherboard problem.
Now I need to find a new one, but my hardware is fairly unmodern (about 3.5 years old). I need to find a board with a CPU socket for a Pentium 4 Hyper Threading (775) and possible support for a dual core for upgrading in the future, DDR RAM (not DDR2), standard (IDE?) hard drive connections. Also support for PCI-E and AGP/onboard graphics, as I wont be able to afford a new graphics card at the same time.
Can anyone help?

I would get a mATX mobo and DDR2, dont waste time, money and effort on those lga775 AGP boards. And no P4 unless you can get a cheap second hand. I know i would go for a Celeron 420 instead, clocked up a bit its quite impressive.

Good thing about mATX is the built in screencard for temporary use + PCIe slot for future upgrade, and then pick up 1 stick of DDR2 now and one more next time the finances allows it. Try to afford a dualcore Pentium E2140 ($80) to go in there.

Before i bought my mATX board (GA-G33M-S2H) i was looking at the megacheap GIgabyte GA-G31MX-S2, checked the manual and it DOES have overclock settings in Bios, Intel chipset for less than $80 that seems like a helluva good deal even if it doesnt overclock all that well.

So basically, its better to just build a new machine from scratch? Excellent, I just needed to hear it from someone else! As long as the machine will accept my older HDD's, and I can still use my existing case (not bothered about flashing lights and see through sides), I'm more than happy to rebuild.

Problem for you could be that far the most newer boards supports only 2 IDE devices. If you have more you can pick up a cheap SATA-IDE adapter, i have used one on intel Sata controller (ICH6 i think it was) and it worked just as perfect as the native Intel IDE controller for both harddrive and DVD.

Welcome to the Forum! http://www.ocforums.com/showthread.php?t=491697
Or get an IDE controller card. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Description=ide+controller+card
So what make/model of hardware do you have now? cpu, ram, psu,etc.

My current hardware is a Fujitsu Siemens Scaleo T. Not sure what brand the mother board is but I see a small hard soldered Intel chip on there so I'm guessing Intel. Processor is an Intel P4 3ghz HT, I think the 1.75gb RAM is by PNY, a mixture of various name brand HDD's 160gb, 60gb & 20gb (Seagate, WD and one I can't remember), and I believe the BIOS is Phoenix MK42i. Graphics card is an AGP Nvidia 6600GT. Windows XP.

Whilst this machine is out of action, I am using an old 800Mhz Compaq, Windows XP with 384mb RAM and a Riva TNT2 graphics card, just about good enough for internet. The problem I am experiencing with this is that I cannot seem to get it to boot from the main 160gb HDD that I took from the broken machine. I have to boot from the older, already installed HDD. However I can still access the data on it when both drives are installed booting with the older HDD. This concerns me somewhat because, when I finally rebuild my broken machine, will it recognise my main 160gb drive as the boot drive?

As for rebuilding my broken PC, I'm not all that concerned at going from 3 HDD's to 2, as the main 160gb one is for all the main stuff like games and system things, and the 60gb secondary is purely for iTunes, other media things and archive stuff. The third one was installed just because I had a spare 20gb drive and didn't want it to go to waste.

Also not all that worried about it being a top flight machine, as long as it can handle BF2142 and Age of Empires 3. So SLI'ing graphics cards and multiple processors isn't necessary. I use my 360 as my main gaming platform.