PSU Kills Motherboard

well thats nothing that I have ever heard of before, are you sure it was hooked up correctly? I mean that is like super overkill for what your setup is but i wouldn't think it would like overpower and wreck the motherboard.

The PC will only draw as much power as it needs to run the system...you won't overpower the system by using a PSU with a higher rating than you need. You'll operate it outside the sweet spot for effiicency, but a non defective PSU won't kill your PC simply by being more than you need. Some older OEM PCs like Dell and HP did use proprietary wiring configurations for the PSU/mobo; it's possible that you ran into this type of issue. Most modern PSUs are designed to fail before they kill the PC, I'm not sure if you'd prefer to kill the PC or the PSU in this case...
What are the specs for the PC you installed this PSU into?

The system i used the new PSU on 'was'..

Asrock P4VMB AGP 8X Motherboard
Pentium 4 (3.0 ghz)
2x512mb DDR1 (Generic)
200GB WD Hard Drive
4XDVD Writer

It's a custom build from years ago that worked fine (aside from the old 230w PSU that was making a racket, hence the upgrade)

Powering it up after the initial 'accident' results in a slight movement of the PSU's fan and nothing else..

Is it safe to assume the new PSU is fine? (it powers up the switch on light and a external hardrive light)

I plugged the PSU into the 20 pin and used one the 4pin plugs for the CPU.
There are numerous 4 pin power plugs that come with the new PSU some i imagine are for powering multiple GPU's (SLI etc..)

If it's a custom build, then the wiring should be fine. It sounds like the PSU sacrificed itself to save the PC. If you don't have a PSU tester like the Rosewill RTk PST: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
then you can try the PSU Paperclip Trick to see if your PSU will run: http://www.youtube.com/watch?v=u0OKmIuNtmI
I would remove the PSU from the System before trying this test and ground yourself for safety.

Well i don't want to run the risk of warranty issues so i won't be sticking safety pins in plugs etc, and with my recent luck i'd probably mess that up and lose my eyebrows or something..

I'm having the offending PSU returned (EBuyer) and i'll just have to get a new system.
Thanks for all the replies. Appreciated.