There's no real way to say how it will react to the 512 stick. Since the limit is 512MB total, that means the limit per slot should be 256MB. If the stick you put in is double-sided, it may only use one side (I've seen it happen before), but else, it might just reject the stick. If you can return the stick for a full refund, there's no harm in trying.
So would you recommend buying this 512 stick: Amazon (only place with a full refund) 512mb DDR 2100
and then seeing if it works?
Because the max is 512, and there is another 128 mb stick, is there a way to disable the second stick in the bios?