Over they years I've bought memory cards from newegg.com, Adorama, B&H, frys.com, Amazon.... and Best Buy locally (once, they were far and away the most expensive, but I needed the card like now).
Figure out what you want, then shop around for the best price on it.
7D can use UDMA type cards. So that might be what you want, and most of those seem to be 266X or faster.
Sandisk doesn't rate their cards the same way other manufacturers do... They probably rate it 45Mb/sec or something like that. (Not 45X) I think that would be approx. 266x or 300x in the rating system that other manufacturers use (but I'm no expert on this, others may know more).
If you read the fine print on Sandisk, they usually show different write and read speeds, too.
But, when it comes to downloading the speeds you'll actually see depend upon what you're using to read the card. To take full advantage of UDMA download speeds you need an UDMA compatible card reader and a Firewire 800 connection, at least. Some built-in card readers (desktop or laptop) may be faster than USB/Firewrire 400, too.
Note: The "rebate" on the Adorama deal is in the form of a prepaid credit card.... But it doesn't appear limited where you can spend it, though, the way some rebate cards are now.
The other question is how large a card you want. Personally I prefer using a lot of smaller cards, instead of one or two really big ones. I have eight 8GB and a dozen or more 2GB presently. Also some older 1GB around here somewhere.
I'd just rather not put all my eggs in one basket, if anything goes wrong I don't want to lose a whole day's (or week's... or month's) worth of images. Not that I've had much trouble with the cards themselves. Only one "glitch" that was clearly "user error" (I opened the door and pulled the card out too quickly). But, cards can get lost or stolen... or left in a pocket that goes through the laundry.
8GB should give you roughly 200+ images with 7D's 18MP sensor (I get over 200 images usually with 8GB cards and 21MP 5D MkII and nearly 300 with 15MP 50Ds... shooting RAW mostly. Less with RAW + JPEG, of course. More with sRAW1 or sRAW2... or just JPEG.)
Depending upon your workflow and backup methods, you might choose a different size card, too. If you burn DVDs, a 4GB card might be perfect, 8GB if you burn double layer DVD... Much larger if you burn BlueRays. Or if you only backup to big hard drives, it might not matter at all.
