Hardware :: White Light Flashing On Monitor / Ram Not Recognized By System

I have a Mac G5 Dual 2.0ghz Running 3GB Ram 6 x 512 ddr pc 3200 400mhz i use it for music production, So as you can imagine i need plenty of ram to run some of the high end plugins, So i purchased 2 x 1GB Pc3200 400mhz Ram, Pushing up my total ram to 5GB, Plenty for what i need.

So i installed the ram in the 2 outside slots(the only 2 left) the mac booted up but wouldnt recognise the new ram,( thought i would save a few pound and went for the extra value ram) So i thought i would move the other ram up a slot and put the new ram in the first slot instead of the last one, it still wouldnt recognise the new ram,So i turn it off took out the ram, put the old ram back in its original slots, Turn the mac back on,......Nothing but a white light flashing on the monitor, & the fans going mad !

After trying each set of ram in pairs one at a time i managed to get one set to work in the first slot, 1 gb (2x512) once! Now i cant get anything to boot this mac up! the ram worked before, so why not now. Ive only been using a mac for about 2 years, and before that i built pcs for a living, & i know my way round pcs fine , but this has really got me.Please Please Please. if someone can help i would be very grateful, i need to get some important work finished.
Best Regards.