remember you will void warranty if you do it yourself!
ive got an imac.. seriously awesome piece of kit.
try either play.com or 7dayshop.com
both have ram for imacs at not bad prices, well not £500 for 4gb anyway!
good luck
I would just get a 2g stick, I have the same Imac as you are getting and it hardly ever uses over 1.5g. But I suppose it depends on what you are going to be doing with it.
you do realise that not all ram is the same, cheap memory is usually not the best try jigsaw as they know what works best 24" 2.8 ghz imac here