Hi,
Having a few problems with my system. Its a Dell, plus one or two additions.

Whats in it @ the mo: SoundBlaster Live, GeForce4, HDD*2, Floppy Drive, DVD-ROM, DVD+CD writer.

I'm trying to put a x700pro (which, unlike the GeForce4, needs a power lead) and an Audigy 2 ZS into it, but if the x700pro is installed, it doesn't boot up.

Now, the odd thing is, is that it worked initally, for a day and a bit, and I was able to run BF2 on it, with no problems. Then, after the day and abit, it just froze. Upon restart, the monitor wouldn't go on. I've checked all the connections (inside and outside of the PC), but nothing seems to be wrong. I've since put back the old graphics card, and it boots normally.

So: my question is: what could be causing this? I've thought it may have been lack of power, but as it has run for a day and a bit, with no problems, I don't think its this. Also, I've tried booting without the optical dives (DVD, DVD+CD writer), but the monitor still won't come on.

Finally, I don't know whether or not its the graphics card, as the odd time (once in 5 try's), it has booted, somtimes only the BIOS screen is shown for 20 or seconds before the monitor goes off, or sometimes it'd get as far as the screen where you'd see the "Windows 2000" logo, and then it would freeze.

Any helpfull hints, or sugestions, would be nice.

New gfx cards, needs a minimum 300W power supply...

Sounds like it "could" be a power problem.If you want I could try the card in my machine and rule that out and maybe try my card in your machine???
I dont think Dell have standard PSUs either so you may be stuck there aswell.

Did you try and boot it with just the HDD with the OS on it and all optical drives disconnected?Also pull out any pci cards aswell to make sure the card is getting enough power.
Did you check the heatsink to make sure the fan is spinning when you turn the PC on??

Tried it on the test machine (478 mATX mobo, 128MB RAM, 6GB HDD*2, DVD-ROM drive) which had a 400WP PSU on it. Still nothing. It didn't even show me the BIOS screen

The fan was spinning, tho. I'll try Valen's suggestion, and take out the CMOS battery to reset the CMOS settings.

Move the jumpers beside the CMOS too SYCO, it resets everything.

Took out the battery, did that, booted it, left it on for 20 seconds (screen was still blank), and switched it off. Put the battery back in and moved the switch back to the "keep data" position, and booted the PC. It showed the BIOS screen, then when it went onto the next screen, it froze and locked up (I left it for a few minutes, but nothing happened, and the keyboard was locked out). Upon reboot, it showed a blank screen.

I waited a few minutes, and then tried again, this time it booted. I went into BIOS, and found out that it only detects a 64MB AGP Aperture size, instead of the 128MB.

In the time it took to write the last paragraph, the PC has locked up again. Thus it seems that with the x700pro installed in the machine, it only lasts for a minute or two, before it locks up, and freezes. The fan from the card is still going, so it is recieving power.