Marc, try MSY in balcatta.
They have the cheapest prices I have seen around Perth.
http://www.msy.com.au

+1 for MSY in Balcatta and also recommend NetPlus in Osbourne Park.

I suggest removing all unnecessary add-ons like additional hard drives, usb devices, additional RAM sticks and expansion cards. Take it down to the bare minimum that will boot i.e. one stick of RAM, the hard drive with the system and the display card.
Failing that reset the BIOS by removing the CMOS battery for a minute and/or shorting the two pins (sorry you will need to check which ones from the motherboard's user manual).
Also check for loose connections with the RAM and display card.

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MY10 WRX hatch, Commodore VE SS, Audi Quattro, Toyota MR2, Peugeot 205 GTi, Mk2 Cortina 1600E, Triumph Dolomite Sprint

Just as Discobay said,

Its most prob the BIOS. Clear it, I had this happen to me few times after overclocking the CPU too much that the mainboard shuts off to preserve the motherboard. Clear the CMOS and then see what happens. 650w PSU is sufficient unless you have more than 1x high-end graphics card.

If that doesnt work, unplug the main power from the PSU, wait like 5mins, plug it bak in, clear the BIOS, then try powering it up. Should work.