Actually, for once the Apple RAM upgrade is arguably a fair(ish) deal. $150 for 4GB installed vs. $145 DIY. Yes, you could keep the old RAM with the DIY, but as noted in another thread the practical value of the old RAM is debatable.
I would expect that the majority--if not all--Stores will do that in-house at the point of purchase. Also, as I recall when I got my iMac G5, I did the in-Store upgrade and got the original RAM returned to me in the box that the new RAM came in.