Memory cards are a commodity product. All CE products that are commodity products leave little room for differentiation and are ultimately prey to rapidly dropping prices and margins. Hard drives, memory cards, etc. are all items that you shouldn't buy more than you need.
I've made the mistake of thinking, "It's on sale...better get lots of memory while it's on sale." As file sizes increase, get bigger storage, but don't get more than you need because it's only going to get cheaper. Knowing I was gonig to buy a 5D soon, I bought a Lexar 4GB 133X card and a Lexar 2GB Platinum card on Boxing Day (Canadian holiday) because they were on sale. I paid $199 for the 4GB card which was down from $299. Well, it's been less than a month and the item has been on "permanent" sale since then.