iMac G4 Memory upgrade attempt 1 - not a huge success

I managed to get a 512mb DDR stick allegedly suitable for the upper (don't touch this because it invalidates your warranty) slot on the motherboard, and put it in to replace the 256mb already there. Result - beeps once, light on monitor flashes some sort of SOS code, does not boot. I'm guessing it's a dud, since the computer was fine as soon as I put the old memory back. However, I cleared out a metric shitload of fluff and dust, so I'm counting it as progress.

later - since the Tiger box said 256mb was OK, I've installed it - took a while, but there were no problems whatever. The only panicky moment was when it asked me for the admin password on the old install - fortunately it turned out to be "admin," my second guess (the first was "password"). I'll leave installing anything else until I've sorted memory.