DDR3 happened. The sweet spot for DDR2 was about a year ago. I got this 4gb Corsair kit for $38 after rebates last November. Same kit is $129 now. I wanted another 4 gb but im not sure that its worth it. I think I might just stick with the 4 I have untill I do a full upgrade.
Edit: OK I reread and you didnt say DDR2 you said memory in general
And actually I do remember reading something about a shortage.