DEll 8400 PSU Upgrade

Hello everyone - new guy to forum, Happy new year -_-

ok i have a dell 8400 this is the spec Click me , how ever i have replaced just about every compaonent apart from the PSU/CPU. I was under the impression i had enough, However it would appear i'm not even suppose to have enopugh to power my RAdion X1950 pro 512MB which needs 500w alone arrr and i only have 350W ..

ok here's a list of the components i have upgrade from original shipment

4 GIG DDR PC2-5200 RAM - Replaced 512MB
160 GB HHD added to enable RAID0 300GB - Replaced single drive
CReative extream sound card - Replaced disabled onboard
Raddion x1950 pro 512MB - Replaced nvidia 128MB
Z-Board keyboard 0- Replaced dell multimidia Keyboard
logitech G9 mouse - replaced dell optical
M1 Killer NIC - Replaced onboard Broadcom NetXtreame Gigabit nNIC
Monitor/modem/router - own PowerSupply

So my question is can/does anyone know of a uk based website that supplys a compatible dell D 8400 PSU (hope fully with out having to hack case to peaces , i have researched on tom's hardware USA but all links are for US sites and i dont want to buy one and have to return it to states if wrong.heres the thread .From my research it's not just a case of picking one up and fitting it, power plug could mis placed or switch may prevent fitment, so i thought it best to ask ppl who may have allready done this task.

I was under the impression that if a PSU was not powerfull enough to power system then system would not boot - am i wrong on this ???,

Or does it just make hardware under perform ???